i dont actually know if anyone saw abrant 3 sizing tutorials on youtube? (well there not there now) but anyway is pretty simple you dont really need all this mumbo jumbo os 72 divied by 86 etc etc (you can do that if you want) but i would give just simple measurement using a tape mesure a go its alot easier
its imple for exaple i will go for say a helmet right roughly the average helmet you get is say about 265mm to about 280mmm that there should already be the perfect size you dont really need to dd 2 or 3 inches because well 3 inches will make it look weird and overly big , all you do is take the measerment of your chin area to the top of your head and only add a few more centermeters to it that there should be perect
hope this helps