this is my first rc plane build. I'm using 4-40 screws, washers, lock washes and blind nuts for the engine mount into the firewall. Is there a rule of thumb as to how much engine offset you get per washer placed between the engine mount and firewall? With such small angles, 1-2 degrees, there's a significant potential for errors in measuring.</p>

Typically one washer to start with, then see how the plane flys.

Hmmm, I never checked to see how much 1 washer will do, I usually use the incedence meeter to check that. On the smoothie, it didn't state or build in the correct thrust? 

it just said 1-2 degrees. Imeasured using rulers and it seems that 2 4-40 washers gave me that, but its not an easy or accurate measurment.

Flat washers for 4-40 screws are close to 1/32" thick.

In theory, for one washer, the offset angle will be:

0.7 degrees if the distance between the center of the screws is 2-1/2"

0.8 degrees if the distance between the center of the screws is 2-1/4"

0.9 degrees if the distance between the center of the screws is 2"

Now, if your instructions give you the angles, you can measure them with an incidence meter or an aluminum angle bolted to the shaft of the engine.

thanks, I should have used trig the first time.

Make sure it's not already built into the firwall. Most kits have it built in

You got me curious, I need to unroll those plans and check it out.