ok i cant find my ashby report i have a30" arrow and the ballence point is at 20" from throat of knoc what foc dose its equate to?  

20/30 - .5 = 16.7% per AMO calculations if I remember the formula correctly

hum i think its more synce its 2/3 forward id think its maby closer to 33% but im just guessin

You are only 5" past center which is 5/30 or 1/6th which is 16.7%

that's the other way to calculate it

In The "How-To" section, there is a great FOC calculator....

ahhhh ok sounds more reasonable thanks. im makin a 1100+ arrow for a friend shootin a 100+ # longbow. just tryin to figure it all out. thanks

DD thanks it workd great it came out to 23.16foc
and 10.92 gpp. thanks for the low down .

funny there r so manny functions of TG i have yet to discover . i love this site

I would have to say that with 1,100 grain arrows FOC is not going to be much of a concern.  That much weight being shot at the speed a 100 lb bow will produce should have enough momentum to punch clean thru anything it is shot into.

im going for armor piercing.  

Stu's calculator has been pretting much spot on for me with respect to FOC when using parallel shafts.

yes but i dont see a catagory for a 2514 loaded with a 2216 bare shaft with incerts and points.

but thanks tradgangers i was turned on to the tg foc calculator by deasertdude and got what im looking for,.  

Here is the link to ashby's report. the AMO formula is on page 11 in a call out box (it is not the only formula, bbut he certainly feels it is the most accurate approximation of the TRUE FOC which unless you are building weapons doesn't much matter).

http://www.tradgang.com/ashby/2007prologueupdates.pdf

I pasted the text of the calculation below:
(1) Measure shaft length; bottom of the nock's throat to the most rearward portion of the broadhead taper.
(2) With tip mounted, determine the balance point by balancing the arrow on a knife edge. Mark this balance point.
(3) Measure balance point distance; from the bottom of the knock's throat to the balance point.
(4) Divide balance point distance by shaft length. This gives the decimal equivalent of the balance point's percentage relative to shaft length.
(5) From this quotient subtract 0.50, the decimal
equivalent of 50%.
(6) Convert the resultant decimal fraction to percent by multiplying by 100 (or simply moving the decimal point two places to the right). This gives the percent FOC.