Winning against all odds :
You are certainly right...most of the time ;
But there are many ( not so many ? ) exceptions like Quatre Bras, some in Spain, Eylau (? ) ... where the cavalry broke the square : sometimes it was formed too late, or the infantry was of poor quality ( bad moral, tired,etc..) or begins to run out of ammo , or the weather was bad : wet powder and short visibility - ...
When infantry was reduced to bayonets, killing a horse could fell him and his man in the defenders and disordered them (spain) so cavalry was fired at short distance (50-30 meters) but not too close, timing being of essence
So, I think it is true to consider - in normal circumstances - that infantry square is safe against unsupported cavalry, but various factors may decrease his effectiveness....So, using percentages ( difficult to compute, I agree ) it is possible to give a variable chance to cavalry .
After all, military history is full of events where David killed Goliath or 400 hoplites broke the charge of thousands ......and that is the best part of wargaming (in my opinion )