Stalkingfox, you did not say what spine you started with. But as Pat said, you add 5# for every inch you shorten the arrow from the original 28". So at 24" you would be adding 20lbs of spine to what ever you started with, assuming you are using 125 grain points.
Pappy, most use the rule of thumb that it is also 5# for every 15 grains over or under that.