This is how it often works for me (might have to to which NPC template I am using):

I put down a lvl 1 NPC.

I raise its level to 20.

It gets too high armor values and I want to lower them.

I cannot lower through writing in the new value in the box. The only way is if I write a "-" before a value in the box, it then lowers the "base value" by that amount.

Yesterday I raised a lvl 1 NPC to lvl 25. Its value went over 20 000 in armor.

The value box won't let me enter too many digits, which means I cannot lower it by more than -9999.

It this how it works for you guys too? Or might it only be with NPC templates from some mod?

Another thing I wanted to do is make a lvl 20 and set its armor to 0, and then use only items to give it a true character value. Can't do that as I cannot lower it to 0.