Here's the code I based it from. It's slightly old but by cmp:
if (weapon_type == 'two_handed' or weapon_type == 'polearm') and (has_shield or mounted):
raw_damage *= 0.85
if weapon_type == 'polearm':
raw_damage *= 0.85
if weapon_flags & itp_two_handed:
raw_damage *= 0.9
Since not everyone is Python literate, here's a rundown:
If weapon is of type "two_handed" or "polearm" and player has a shield or is mounted he gets -15% damage
In addition another -15% (total of 1 - 0.85^2 = 0.2775 = 27.75% reduction) if weapon type is "polearm" or another -10% (total of -23.5%) if weapon flags contain "itp_two_handed"
I have no idea what's the flag itp_two_handed or if I misread some other part of that somehow - I have very little experience about Warband modding - but:
Shield and/or Mounted + Pole = -27.75%
Shield and/or Mounted + Twohander = -23.5%
Source:
http://forums.taleworlds.com/index.php/topic,168722.msg4090900.html#msg4090900Now, the numbers may be just plain wrong. Feel free to correct them if you got good source for proper numbers