oh, ok ... sub (50%) with flanking I (+10%), II (+20%) and Tactics (+30%) ... that is 110% chance of withdrawl - nice.
edit: duh - a trick question?
GlobalDefines.xml has
<DefineName>MAX_WITHDRAWAL_PROBABILITY</DefineName>
<iDefineIntVal>90</iDefineIntVal>
so you can't give a unit a promotion which would push it over the 90% (Max is Sub with Flanking 1 + Tactics = 90%).
Still good enough for a lot of XP milk.