Decide on your thrust angle (2 or 3 degress - your choice).
Call this angle A.

Measure from rear of the motor mount forward to the engine prop hub.
Call this distance D. In trig terminology, this is the hypotenuse of the right triangle.

Use your calculator to calculate the sine of the angle.
Call this S.

Multiply D times S to get the offset to the left. The offset is the side opposite the angle.

Example:
Prop hub is 6" in front of engine back-plate.
Offset is 2 deg.

A=2 deg
D=6 inches
S=sine(A)
=sine(2 deg)
= 0.034899

Offset = D * S
= 6 * 0.034899
= 0.20940

So you offset the engine mount 0.209 inches to the left in order for your right thrust angle to center the spinner in the thrustline. That's about 7/32"

I hope the math wizards will review and correct or confirm this. I wish I had Minnflyer's diagramming expertise. There is a really simple excel spreadsheet attached to calculate this. Enter values in the yellow cells.