Perhaps the value is too high, and thus "wraps" back around to 0 or something similar. Have you tried attacking with the item to see if the knockback is indeed stupidly strong or broken as the tooltip suggests?
Well,I already tired to change the value to 20 and the knonkback was still extremely weak.
And no the knonkback of my laser pickaxe is not as strong as I wanted it to be,it's broken as tooltip suggests.