From: Leeuwarden, Netherlands
Depends what you call success; total destruction, forced the opposing unit to retreat or just inflicting casualties.
There's .. IMO, no precise way of calculating the odds of success, I do it by a hunch, what do I think the odds are. First I check the opposing units, how well are they dug in, how many troops are there, how "green" is their strenth bar? Personally I tend to tread lightly, if there's any chance the opposing unit may inflict heavy casualties on *my* side, I just take my time--->encircle if possible and take a few turns to bleed them dry, bomb them meanwhile.
If the opposing unit is say, ripe for the plucking, undermanned or with a yellow/red health bar, then I attack with my divisional forces combined (if the subunits of the same division are next to the attacked hex they give me that divisional bonus) or even with a concentric attack bonus. That should give me better odds, though that doesn't always result in the destruction of the opposing unit, sometimes just a retreat, or you inflicted heavy casualties.
And ofcourse, panzer units rip through inf divisions.
Maybe Barthheart has something to say about this?