I guess you are building a kit? you have it right. draw a centerline on the stab and elevator for the hinges. mark off the positions of the hinges, usually 3 hinges to an elevator half, for example. use a #11 blade to cut slots for the hinges, they will be a little over 3/4" to accomadate the width of the hinge and about 1/2" deep to accomadate the depth of half the hinge. You can use a spare hinge to check the fit. Draw a line on the hinge at the mid point (this point would fall between the stab and elevator half if you glued it in) so that you can check the depth of your slot.

The best way to draw a centerline is to lay the part flat on the table, lay a pencil flat on the table and run the part along the pencil point. use playing cards and spare balsa, etc, to adjust the height of the pencil and part as needed.

peter