Is some right thrust built into the firewall, or are there instructions regarding right thrust? If not built into the firewall, then some washers under the left side of the mount will establish the thrust line.

You have to mount the engine with a slight left offset so that the propeller/spinner will end up centered where it exits the cowl (i.e. the fuse center line)

If the right thrust is built into the firewall, and if the distance from the back of the engine mount to the propeller is "x", and the thrust angle is "theta", then the left offset along the firewall that will center the propeller is: y = x*TAN(theta).

If the firewall is perpendicular to the fuselage and you'll and washers to establish the right thrust, then my geometry shows the offset to be y = x*TAN(theta)/COS(theta).

In both cases, since we're talking about small angles,
y = SIN(theta) is an adequate approximation.

Since none of the construction is very precise, the last is fine in all cases.

I gather you've got a scientific calculator or some old sin tables lying around?

Scientific enough? It's high school geometry