Formula for determining spine wt.

[snag]

I read someones post that stated as follows for determining spine weight to be used with your bow: Bow weight X 1.4, add or subtract 10% for each inch over 28" or under 28", also add 1# spine wt. for each 5 gr. of tip wt. over 100gr.

This means if I am shooting 51#@28"=
50#x 1.4 = 70.0#add 10% or 5# =75# I shoot 160gr field points, that's 60gr over 100, so add 12# for a total of 87# SPINE WEIGHT for a 50# bow!!! Am I doing something wrong here or is this just plain NOT a workable formula. I have been making arrows spined 55-60# and cutting them to 29" with 160 fps. They bareshaft fine so I guess that's what is important. But when I read about a formula and is suppose to tell you what your set up should be and it is off by that much it got me second guessing.....?

[Bjorn]

That formula looks like the one on the Morrison site but someone left out a few numbers....... If you go to the 'tuning section' in the Morrison site you will find the correct formula there and it works for me. OL Adcock has a very complete tuning section on his web-site that includes bareshafting so visit there too.

[snag]

I guess I'm missing something...On Morrison's site they figure 90-95#spine weight arrows for 58# bow!!!!! That's with only 170 gr broadhead!

[Bjorn]

My 50# Arapahoe takes 75-80# and my 48# Adcock takes the same. That is with a 145 grain tip and the arrow cut to 28.5 BOP. The formula works for me.
If you are bareshafting fine then just continue to do what you are doing. These numbers can vary depending on your bow, like how deep the shelf is cut relative to center, and even the diameter of the arrow shaft.