*offending dragon speaking* Discipline checks are never shown in combat for knockdowns. It is a standard attack roll.
Quote:
Knockdown
Specifics: A character with this feat can attempt to knock his melee opponents to the ground. The character makes an attack roll at -4, and if successful the defender makes an opposed discipline check against the character's attack roll. If the character wins, the defender is knocked to a prone position.
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A character can only make a knockdown attempt on opponents that are one size category larger than himself, or smaller. The opponent gets a +4 bonus on his opposed check for every size category he is larger than the attacker, or a -4 penalty for every size category he is smaller.
Improved Knockdown:
Specifics: A character with this feat counts as one size category larger when he makes a knockdown attack. All other knockdown conditions still apply.
Size huge give +8, IKD pops that to +12, so the base DC for my first attack of the round is 52+d20. That being said I do stealth at -8 as a dragon. This -8 is not shown on the character sheet, but is known.