Don't understand the "drill off center"??? Are you worried about
the washer not sitting flat against the rear of the motor mount
surface?? If that is it, you are trying to be more precise than
you need to be. Just put the washers in place and run the motor
mount bolts through them. What the kit builder is giving you for
thrust is just a starting point anyway. Final thrust settings (up/
down/side) are only finalized in flight.
Bruce