Ya you can do it that way or make a perfact hole just by:

Takeing a small peice of old lexan about 3/4 of an inch long or so, drill a hole on the very edge of each side, then place a screw through one hole, and simply screw the peice to the windshield were ever you want the air hole, then simply stick your exacto knife in the other hole then start going around in circles till theres a nice deep line, then just use the exacto to cut the rest of it out. Takes no time at all, and the end resault is a perfect circle!!