There are a lot of ways to transfer layout lines from the inside to the outside, and a lot of times you need to do so. While it's possible to use reference areas like a wall or header and measure from there, I've found a faster and more accurate way is to drill a hole straight through using a stiff wire. The benefit of the wire verses a long thing drill bit is that the wire won't pull material through the hole (like fiberglass insulation, for example), and won't create a mess to clean up. Check out the video to see how I used this method to locate two dryer vents so that they would align perfectly with the bottom of a course of shingles.
Next Level Carpentry