ORIGINAL: Missileman

Using the drawing r2champion posted, along with a compass, protractor and ruler you should be able to figure it out.
Remeber, however, most airplanes require some offset thrust and the engine in alot of cases are not centered on the firewall but are slightly off to compensate for the offset and still have the engine exiting the cowl properly.
An example: I am curently building a Norseman. It requires 2* right and 2* down thrust. In order to have the engine centered in the cowl it has to be 1/8" above center and 1/8" to the left of center on the firewall.