For best results, you want to rank Critical Hit in conjunction with Windmill. With both at rank 5, I can deal over 400 damage on a critical windmill. Windmill ignores enemy protection when determining critical chance, and if it scores a critical hit on any enemy it hits, ALL of the enemies in its radius take critical damage. Also, both skills give lots of Will, which boosts critical chance.
As for what rank is "good enough"
For use as an escape button or for those who N+WM, R9 is "good enough"
For those who want to totally decimate rooms, or who want to get through Baol Final, R1 with R1 Critical Hit is as good as it gets (though in the case of Baol Final, if your two partymates have R1, you can get by with R5 or even possibly lower if they're totally uber).
