Its just the percent of weight forward. I simply find the fulcrum, measure it to the midline and find out the percent of mass that is forward. I like a higher foc for penetration. I use stiff shafts with lots of head weight. I do it for the quite shot and penetration. A higher foc can help the fletching stabilize the arrow a bit.

Low foc is ok with compounds, like 10-12%. But with a trad bow, I like to have much higher. Ive not calculated foc in a long time. Simply because ive had no problem with penetration. Ive stuck with a 650 grain arrow and 145 grain head. But since you mentioned it,I might have to figure mine, just to see what it is. Im like you, when it aint broke, I dont fix it.