I believe there are several numbers that could tell you more:
50 percent - when strength is equal
66 percent - 10 percent advantage
73 percent - 20 percent advantage
78 percent - 30 percent advantage
88 percent - 40 percent advantage
I your unit is weaker, just reverse situation f.e. AI unit is stronger by 20 percent - AI unit has 73 percent chance to win.
So the full table is as follows (it will be helpful for my purpose) ...
12 percent - -40 percent
22 percent - -30 percent
27 percent - -20 percent
33 percent - -10 percent
50 percent - when strength is equal
66 percent - +10 percent
73 percent - +20 percent
78 percent - +30 percent
88 percent - +40 percent
If you have unpromoted Harcher and it has 22 percent chance to win, promoting him to Combat should give you 33 percent chance to win and 66*0.2= 13.2 percent retreat odds. 46.2 odds altogether
Promoting the same unit to flanking2 should give you 22 percent chance to win (plus immunity to FS, which is not considered in my model) and 78*0.5=39 retreat odds. 61 odds altogether.
66*0.2 means 100%-chance to win, in this case 33 percent is chance to win, while 0.2 means 20% - base retreat chance for every horse archer
78*0.5 means 100%-chance to win -22 percent, while 0.5 means 50% - expanded retreat chance - 20% +30% (from flanking promotion).
Generally, I wanted point something. And it is that 33 percent, despite it's not very friendly result, it means that you are getting close to outnumber enemy. So promote to Combat2 then or use catapults.
Another thing I wanted to point is that Combat is most fair promotion. Because promoting to Cover, Shock, Pinch, Formation, Charge will never give you desired 25 percent advantage. It will give you 25 percent advatage (according to my table) only in one particular case - a) defender has no bonuses. b) only when you attack. Usually it's much lower bonus. It's better sometimes to promote unit to Combat rather than City Raider, because Combat will give you greater odds.
Pikeman attacking Knight has different odds than the same pikeman defending against the same knight (in open ground of course). It's wierd I know.