If you've ever installed a lockset or deadbolt on a door, you know how hard it is to accurately mark where the strike plate goes on the door jamb. The solution is to use a little lipstick.
First, open the door and rub the lipstick onto the latch or deadbolt. Any color will do, but bright red works best. Turn the knob to retract the latch, close the door, then release the knob just enough for the latch to "kiss" the jamb. The resulting imprint indicates exactly where you need to mortise in the strike plate.
If you've run out of lipstick (hey, it could happen), mark the latch or deadbolt with either paint or chalk.