Yeah, dealing with necromancy turned out to be more complicated than I thought. I placed 4 heroes with different skill levels on the map and put 100 goblins in front of each of them to be slaughtered.

In the unmodified version, heroes resurrected twice as many skeletons as they should have – 40/60/80/100.
In the modified version, complete chaos ensues. I changed the data so that 10-15-20-25 skeletons are resurrected (5% base skill and 5% per level), but the skeletons are resurrected as 16-24-32-40 (i.e., a 1.6x increase).
The last version I made with these values was: 1% base skill, and 3% per tier, resulting in 6-11-16-20 resurrected skeletons. If I set the base resurrection to 0, then 4-9-14-19 will be resurrected.
The same nonsense is happening with the numbers for the Necromancer's staff, the Amulet, and the Death Lord skill.

Well, that's okay. It turns out that Heroes has a system for balancing the resurrection of the dead, meaning that when skeletons are resurrected from Tier 2, the number of creatures resurrected increases by 2 times, Tier 3 – by 3 times, and so on until it hits the ceiling. I wanted to resurrect hordes of skeletons from low-level units, but in the end, I end up resurrecting more from Tier 3... Tomorrow I'll try playing with a resurrection of 6-11-16-20 without dark energy; let's see what happens. If I don't like it, I'll revert everything to how it was and make the resurrection level-based.

Added after 16 hours 34 minutes
It seems I'll have to revert to the system of resurrecting mixed-level units. I don't like that 5 skeletons are resurrected from 50 peasants, but 15 skeletons are resurrected from 36 possessed. This destroys the whole concept of cheerfully slaughtering peasants