If you need to use the "eyeball" method, try this.
Assemble the mirror on the bracket. Starting on the left side, sit in the drivers' seat and with the assistance of a friend, position the mirror and bracket where it works best for you. After you find that position, mark the three mounting hole locations.
If you are mounting a RIGHT side mirror,,,
THEN, with a pencil, mark reference points on the door to take reference points. ( distance from leading door edge, distance from window opening, Etc.) Then use those measurements and transfer them to the passenger door.
If you get the forward hole position accuratly, you can use the bracket gasket to mark the other two holes. It's important to get the right side in the same position as the left because, from the front, it will look lop-sideed if you are off.
Good luck !
DVal