You can't. More attack=better chances. How much more ? Nobody seems to know. I can tell you that for a 40 defense planet with 205 attack and no other bonuses I had 60% chance of conquering it.
Same goes for combat. Nobody knows how it works

P.S.
Obviously, nobody but Dan but Dan is very secretive about it

_________________"Life is not that complicated. You get up, you go to work, eat three meals, you take a good #&$# and you go back to bed. What's the f@#$ing mystery ?"
