I found Nethergate's documentation for the effects of armor encumbrance and Armor Use skill to be misleading (as of v1.0.2 for Windows). After some experimentation, I figured out the correct numbers.
Claimed:
C1. Some armors give a penalty to hit, in increments of -5% (up to -35% for Chain Mail).
C2. Each level of Armor Use cancels -5% of penalty.
Using these numbers in my party spreadsheet caused consistent discrepancies, compared to the actual %hit chances printed during combat. So I did some testing.
Actual:
A1. Define a new concept of "encumbrance level". Convert every written "-5%" to 1 enc level. So a helmet ("-5%") really means 1 enc, and chain mail ("-35%") really means 7 enc.
A2. Define a new concept of "penalty level". Convert enc level to penalty level 1-for-1, except that you must add 1 "bonus" penalty level(haha) at enc levels 3, 5, 7, and 10. (In other words, 3 enc = 4 pen, 5 enc = 7 pen, 7 enc = 10 pen, and 10 enc = 14 pen. 11 enc is the max possible that I know of, using chain mail + large shield + helmet.)
A3. Each level of Armor Use cancels 1 penalty level.
A4. Each penalty level gives -6% to hit (not -5%).
A5. Every 7 penalty levels also gives -1 AP! (rounded down)
Hence, if you wear leather armor (2 enc), a metal shield (2 enc), and a helmet (1 enc), you have 5 enc = 7 pen.
- With Armor Use 0, you have -42% to hit, and only 3 AP.
- With Armor Use 1, you have -36% to hit, but at least you get the full 4 AP.
- With Armor Use 7+, you have no penalty to hit.
A Celt sword guy maxes out at 5 pen, for a large shield (3 enc) + helmet (1 enc), so he could reasonably need Armor Use 5. A spear guy only needs Armor Use 1 to cancel a helmet, which you'd get automatically from having DEX 4. A Roman (including Numerius in a Celt party) could go as high as 15 pen for the full 11 enc, so you'd need Armor Use 15 to be unpenalized, or at least Armor Use 9 to get all 4 AP.
Penalty levels are probably -6% to give you incentive to train Armor Use, instead of just dumping more levels into Melee Weapons or Spears.