Spell Percentages

Ok, probably a dumb question...When using a temp Hero & adding levels to various spells, I notice the % numbers becoming a negative...& rather large ones at that.. What, exactly, does that mean?
Thank you for any response.

There are no dumb questions (just bad answers!)

That means you have a very low probability of casting a spell in a high level. Say you have 100% for level1; 40% for level2; 20% for level 3; -30% for level 4; -70% for level 5 etc.
that means you are trying to develop powerfull, great spells for your hero, but he/she cannot do it yet (probably because you forgot to develop intelligence as well?). someone here may have a more technical answer for you

High level spells have low spellcasting chances. Negative spellcasting chances means those levels of spells can never be cast. Look for a way to increase your hero's spellcasting chance (intelligence, perks, skills, items, researches) to use such high spells. Remember: the higher your spell level, the lower the spellcasting chances.