Yup, I agree with mace, I would go for half flap as servo centre, and work from there. You then have end point adjustment for the up and the full flap positions using the matchbox.
These should be VERY close to each other if your mechanical setup is good. You may even get it all done on the Tx without needing to go to the matchbox at all.
Good luck and enjoy the new smmmooooottthhh flap deployment.