I gap off the riser. More specifically, I gap off the top of the strike plate.
It seems to work for me at this stage. Just offering how I do it.
I shoot 40-50 lb draw weights at my draw length ( 29.5-30"), with a 3 under Bateman tab. I shoot local 3d, and I hope to deer hunt this year.
I shoot xx75 Easton aluminum arrows. Mostly 1916 and 2016 left full length. My points are 145 grains. ( sometimes 170 grain, sometimes I play with 125 grain).
I set my bow up for a "zero gap" off the top off the strike plate for 20 yards. I have 5-6 recurves and 1 R/D longbow. The height of the strike plate above the arrow is just about 1 inch. This gives me my "elevation reference." For a 20 yard shot, I hold it just where I want it-POI is level to top of strike plate. Left/right is determined by looking down the arrow shaft (not at the point-I don't really see it (or look for it), as I don't gap off the arrow tip/shaft end-just off the strike plate top.
In essence there is an invisible rectangle formed that is my "sight." It is vertically above the arrow shaft, and horizontal with the top of the strike plate. I can play with this, and being right handed, if I put a heavier point on, it makes the shaft weaker, and impact lower. I can manipulate this enough to have the impact point nearly ON the top of the strike plate-but I prefer the "invisible rectangle" approach directly over the arrow.
What is interesting is that if I shoot further away (~27 to 30 yards, about the traditional/barebow max distance at my local 3d club), I hold over about 1 inch for every yard over 20 yards, so I'd hold 5-7 inches over the desired impact point-using the top of my strike plate as my elevation reference @ ~ 25-27 yards. On a deer-sized target, I hold the top of the strike plate on the top of the target, and the arrow falls into the "high point area" (at least it's supposed to). I have to add a bit more elevation with 2016 arrows at 25 to 27+ yards as the arrow is heavier and slower, and doesn't shoot as flat as the 1916 shafts.
On shorter shots (~ 12-15 yards), I hold UNDER about 1 inch/ yard less than 20 yards. So, at 15 yards, I hold the top of the strike plate about 4-5 inches lower than where I want to hit. On a deer-sized target, the top of the strike plate is on the bottom of the body.
I also adjust my left/right with the thickness of the strike plate for each bow, (plus bare shaft tuning for weak/stiff with different arrow tip weights.)
I bare shaft tune for nock height. I like to run my nock as high as possible, as long as I get good arrow flight, because this helps to match the impact at 20 yards with the top of the strike plate level. First I find the point were fletched and non-fletched arrows have similar impacts (high/low leveling out), then I bring the nock up to get the impact level with the top of my strike plate at 20 yards. My typical nock height above center is ~ +3/4" )5/8 to 1 inch).
I can also adjust the impact height a bit with my anchor point (I mean, we ALL can do that-nothing new there). So, there is quite a bit of adjustment available to make it work/tune it /"sight it in" for 20 yards.
Sometimes a heavier arrow just works better for this. I have a 64" Samick Journey with 35# limbs, that I pull 42# @ 30 inches. It shoots well with both 1916 and 2016 shafts (I wrote about it in the tradgang product bow review section as an reply to another persons Samick Journey's "review in progress"). Overall, I can shoot the 1916s with 125, 145 and 175 grain tips and do well. 2016s with 145 grain tips fly well, but shoot lower )that's ok with my style).
I have a 1959 Bear Kodiak Special 40 # / amo 68" that pulls ~ 45# @ 30", and it responds well with 1916 and 2016 xx75 aluminum shafts using gapping off the top of the strike plate-so it's not just one bow. I have a 1959 Bear Polar (47#@ 28" 64" amo) that is set up the same way. Shoots 1916 and 2016 well. So, all these bows are set up to shoot almost the same. It seems to work for me at this stage of my shooting.
The other reference point I find for each bow/arrow combo is the point on distance. It seems to be about 33-36 yards. Sometimes the compound guys chide me. We shoot weekly at our club in a fun league, and I'm the only recurve guy. They shoot from ~ 32 to 40+ yards. They joke that it's easier to hit the target from the 27 yard (or closer) stake. I offer them my bow to "show me how easy it is." No takers. I can then step back to their ~35 yard marker, shoot point on distance, and do pretty well. Often I can land an 8. In some ways, it's easier, as I do use the point as the aiming point.
Anyway, it's good to know the point on distance, even with gapping off the top of the strike plate.
I'm no expert. I'm not at great shot. I'm just offering what I do, and how I do it. I find it rewarding and fun. I shoot in my backyard just about daily, and try new things and reinforce other things. I am averaging 7+ points on my 3d scores this year (10-8-5 scoring), and my average is coming up.
( UPDATE: Shot a 30 shot 3d competition this past weekend ("all deer targets" , so granted bigger scoring areas for higher scores...). Finished with a 260 ( 122 on front 15, 138 on back 15). avg = 8+ Used my stress-cracked 1959 68" 40# Kodiak Special with 1916 full length xx75 shafts and 145 grain points. Used this method of shooting. Set personal highs at the shoot. Distances were ~6 yards to 28+ yards).
So far, I like the concept of holding either to the top or the bottom of the deer's body for the impact area. It narrows the aiming area for me. Hopefully that will help during hunting season.
Now, all I have to do is make my release uniform...
Videos that have helped me: Jimmy Blackmon "Gap compromise" (and others). DeWayne Martin and high nock height, Arne Moe form videos, Wedge series, Jeff Cavanaugh, Wolfie Hughes, and others.
I tried gap shooting, using the end of the arrow shaft ( or point) as the reference, but I didn't really like it. I wasn't "seeing the target", just the dirt/leaves/ hoof. Gapping off the strike plate allows me to see the target, and use it as a reference for height adjustment. My reasoning for liking this more is that if/when I hunt with my bow, I will see the quarry "better", and be more aware of what it's doing. (Just my theory).
If you are NOT shooting IBO ruled 3d, and "just hunting and recreational shooting, you have more options with strike plate height. Higher strike plate reference will "lower" or close the gap off the top of the strike plate. Try something like 1 1/8" to 1.5" high strike plates. You can also set your reference distance / zero gap @ strike plate top for something like 25 yards. Use the best set up for your needs.
Good luck.