I am new to the tec. part of bow hunting. I only heard of F.O.C after reading post on this site. I looked at gold tips web site at the calculator but did not know the right weights for my arrows. Iam shooting a28.5 in. super carbon magnum 55-70with 4 in. feathers and 100gr. thunder heads. The knocks come with the arrows i don't know what brand they are, but I'm going to try lumanocks this year. Can any one help with this???

I can't help with the weight of your arrows. I will say the FOC on them is probably in the 10-12% range (might be higher), given 100 gr broadheads with feathers on a 28.5" arrow. FOC is the percentage of weight forward of the center of the arrow. With broadheads, 10% is generally considered the minimum. You should be fine with your setup.

It's easy to figure yourself. First, make a mark in the exact center of your arrow. Next, take a butter knife and find the balance point of the arrow. Mark that spot as well. Measure the distance between the 2 marks and divide by total arrow length. Multiply by 100. Done.

MoBow said it. Simple as that and better than a calculator. I measure the whole shaft from nock end to end of tip or head. Others do it differently.