I had to make a new firewall for an arf I damaged in a crash. I decided to mount the engine inverted this time, so I had to do everything from scratch. First I found the vertical center by measuring the midway point, then drew vertically with a right triangle/square. . . I then determined the distance from the firewall to the opening in the front of the cowl. Using a contractor's (home depot) protracter, I determined I had built in 3 degrees of right thrust in order to center the crankshaft in the opening of the cowl. . .I multiplied the tangent of 3 degrees (scientific calculator) by the distance from the firewall to the cowl opening (say 6") this told me how far to the right (looking at the front of firewall) to make my vertical cross hair. As far as finding how high to mount the engine on the bracket, I put the cowl on the fuselage temporarily and taped a piece of straight cardboard along what would be the centerline of the cowl, I then moved the engine up or down as necessary, and drilled holes.

With the template made earlier he would have used a circle template (you can get them at office depot) used for drafting. . . the circle templates have crosshairs on them, so basically find a circle that matches, lay a straight edge over your crosshairs on the template, and draw them inside the circle. Taddaaa. Hope it helps