hi good question, think about it to the extreme,imagine a 1 pound arrow head on a skinny shaft.it will bend and snap ..you got to get the right balance,.....no arrow head needs a weak spine,,,,,lol,,,,,,,,A 1 ton arrow head on a 8mm oak shaft,will snap and land on your toes.my only advice is trial and error .its a very good question that needs to be rectified. personally i try and make my heads all the same weight and size,the rest is just guess work...and excuse me i am pissed,6 pints of cider.....goodnight,..