FOC% stands for forward of center percentage

is the amount of weight that is in front of the center line of the cut length of the arrow.

FOC is calculated most precicely by Finding the balancing point of the arrow by trying to balance it from a nail. once you have balanced the arrowMark this point on the shaft.

then with a tape measure measure from the throat of the nock ( where the string fits up aginst the nock) to the end of the shaft, not the end of the insert but end of shaft, this is the cut length of the arrow
divide this measurement by 2 then mark the center.

measure the balancing point to the center mark, multiply by 100. then divide that number by the arrows cut length
this will give you the arrows FOC%

you dont want to be under 7% nor over 17%

I try to stay in between 8% and 10%

now spine needs to be determined by how much peak draw weight your bow has and your draw length
It is better to have a higher spine deflect than a weaker spine
the draw force curve on the cam also will determine whether you need a stronger spine
If you have a bow with a hard cam you need a very stiff arrow
a mild cam does not require as stiff of an arrow.
BUT IMO if you are shooting a release and using a loop and youre bow is well tuned there is no such thing as over spined although some will try to argue that that is still my opinion.

nevertheless you are better off over spined than under spined.
Hope this helps.