So, I finished as a mage build recently. Thought I would share some observations and see what you think.

Sign intensity
- Final sign damage = (base damage * sign intensity) + bonus sign damage
- Aard has a base damage of 0 and Igni has a base damage of 10. Both values are unchanged in upgraded Signs.
- Hence the sign intensity's contribution to the final damage is smaller than I previously thought.
- As for the critical effects, I'm not sure if they multiply the chances by 2x or 3x (with higher Sign intensity).

Vigor regeneration
- The formula seems to be:
Total regen = ( base rate + rate from armor + rate from intoxication (condensation skill) ) * skill / potion modifier.
- Geralt has a base rate of 0.10. Say he
a) wears Ysgirth armor (+15%)
b) has Condensation II (+40%).
c) has Vigor Regeneration II (+25%),
d) has Fortitude (+10%).
e) has Schemer II (+40%).
f) drinks Tawny Owl (+20%).
g) drinks Lapwing (+50%).
How the math works out is:
Total regen = (0.10 + 0.15 + 0.40) * 1.25 * 1.10 * 1.40 * 1.20 * 1.50
- In short, if you don't have equipment that +Vigor regen or Condensation skill, it seems like all these other + Vigor regen skills and potions don't really do much.

Hope it wasn't too much math. Anyone who specialized in Signs can comment on this?

I have to more or less agree with the vigor regen thing.
I found a huge difference when I put in the kayran skin enhancements, though it's only 2x +5%.

I think the Kayran skin enhancement belongs under the skill / potion modifier. So if you have 0.10 base only and nothing else, the resulting regen rate with 2 pieces of kayran would be 0.11

Really? Hmm.
Does the vran armor add vigor regen?
Can't recall why else it would be so much faster.

Don't think so. I think I shall double check what exactly the kayran enhancement does. I have only used it on Ysgirth so there might be something funky going on.

I might have to load up my save to see if there's any other bit of equipment that adds regen, too.
I think I replaced every piece of armor simultaneously.