no formula...simply what you want from your bow. for me i an a hunter. i shot more targets than alot of hunters..but thats all just practice for game day. as a hunter i want maximum KE. but to get the maximum KE you need to figure out the balancing point between speed and arrow weight. speed and mass=KE. (.5)MV^2=KE. so velocity has more importance than mass..but without mass you wont have alot of KE. theres a balancing point for it where you gain the most KE at a certain speed..adding anymore wieght slows you down and only raises KE verrrry slightly. what i did was find that point. i now for my setup i want to be about 450ish grs. but you gotta figure out how fast you will be shooting diffrent wieght arrows...i did that by knowing how fast i shot my current arrows..then figuring out the speed of diffrent gr arrows...then running the KE calculations for diffrent arrow weights...takes some work...but its worth it IMO. ive always liked numbers and math..i got bored one day and did it lol.

but with all that being said...most of todays bows will kill a deer..PERIOD. you dont need 60-70ft/lbs of KE to kill a deer. they say something like 35 is enough...but the higher your KE the more power your arrow has behind it..blow through bone and tissue easier...

heavier arrows are said to be quieter and will be less effected(still effected though) by wind...and will carry their KE better than a lighter arrow with less weight..

the determining factor is YOU..what you want..lighter arrows=more speed which will give you a flatter trajectory..less of a pin gap etc..