If you are a zerker, then 70/99/45 would probably be the best. However since you already have 75 att, 99/99/45 would be the next best. If you don't want to go passed 75 att, then 75/99/45 would have to do.
You can do it with 75/90/45, however it isn't as ideal as what I said above.
I personally not a fan of zerks low defense can be so random, my current staker is 99 hp, 85 att, 75 def and I do very well with that. That is if you can wait for your odds, if you are impatient your better off doing just maxed main staking, that way you not giving odds away at least.

