I just built a set of tapered arrows for my son. My tester has a 26" span. I put the nock end of the arrow right on one support and, basically measured the last 26" of the arrow.
I don't think it matters too much as long as you do each arrow in the batch the same way. Consistency is what matters.
I started with the shafts slightly stiff according to Stu Millers calculations. Then I fletched half of them. I'd shoot a couple of groups and sand the arrows, testing the spine every time, until at 7 yards I was getting a both bare shafts and fletched shafts to hit in one group. When I sanded and tested, I made sure that each arrow spined within one or 2 pounds with the index of the nock pointing up (just another consistency thing). When I put the nocks on I tried to line the stiff side up with the index.
Last night we took them out and shot them at longer ranges and the arrows were right on, just needed a little brace height and nock point adjustments and we were all set.
This was the first set of arrows I have made actually using the spine tester, so I am not necessarilly an expert. There are probably better ways. I just tried to treat each arrow exactly the same way at each step.
I put my drill in a vice, then chucked the arrows in the drill and did my sanding that way. I don't think the tapers are all exactly the same. I didn't use calipers to check it. However, that might be a good thing to check. I just thought the spine was more important thatn the taper.
Good luck and good shoot'n,
Doyle