Based on what is written, 3 points will give a 15% damage increase.
It should be clarified that I am calculating this for two stacks of 100 skeletons each. One Gaaltranian with +3 attack modifier and +2 armor (= approximately +20 damage)* and the second Vidomina (+20% to quantity = +20% to damage). In Physmig, in the chapter "how necromancy works," it says that an expert can resurrect up to 30%. I took 20% because I am doing the calculation for level 10-12 characters.
*this modifier was taken based on the post /post/84042/, which was accepted as true since no one disputed it further in the thread.