This is what I do.... Lean the bow aginst the wall. Back up a few feet and align the string with your eye where it runs up and down the center of the top and bottom limb. As your eye follows the string up and down to lign it up center notice if the string clears you side plate (or riser if you have no side plate yet) or not. Cut TO center is not center after you install a side plate. Very few bows are cut past center but some bowyers cut 1/8th past center so when you install a side plate it will be center cut. This technique works very very well.
If you have a front mount takedown then they make a compound center tool you can buy to tell if your center or not. It plugs into the allen holes of the limb screws.
Now for my opinion. Its good to know the exact spine of your arrow. Its a waste of time to think you can calculate what spine you need without shooting a few different arrows and seeing what works best.
here is where I usually start.
Assumptions are that you draw 28"
bows 42-43 pound or less 30" arrow .600 spine with 75-150 grain tip
44-53 pound 30" arrow .500 spine (gt 3555) with 100- 150+ grain tip
54-63 pound 30" arrow .400 spine (gt 5575) with 100 to 150+ grain tip.
63+ pound bow 340 or 300 spine (gt 7595) with who the hell knows... I cant pull a bow this heavy and if I needed this much weight to kill something I will use the bow for firewood to keep warm at night and hunt with a gun.
Remember fred eichler took the super slam with
[email protected] so if you draw 28 thats probably a 58-62 pound bow depending on your bows speed.