It needs only one male in a 400 meter radius that might pollenate, to ruin your female buds. That's what i've read. Destroying the male is a must, i think, because even if you keep your male in the cellar and the females in the attick, you yourself might accidentaly pollenate your plants.
That's why god created animals
When the preflowers exactly open, that i don't know. The more experienced guys will surely tell you
peace