Engine offset
Most model kits have engine and thrust angles designed in (they angle the firewall) to help offset engine torque and help the airplane track straighter down the runway and in the air!
Well you guessed it, there is a formula to calculate both should you need to ADD your own offset or repair firewall damage. The formula for measuring the engine offset is: Engine Offset = (engine length) * sine(thrust angle)
Here is an example : the length of the engine from the back of the mounting plate to the back of the spinner is 6.5". If I want to have 2.5 degrees of OFFSET then: Offset = 6.5" * sin(2.5) = 0.2835". This is approximately 9/32.
Which means that to have 2.5 degrees of engine offset, you will need to move the centerline of the engine 9/32 to the RIGHT (when looking at the firewall) for mounting so that the center of the prop is in line with the center of the firewall (or the manufacturer stated centerline).
Thrust angle
How do you find out how many washers you need, or how much longer to make the left spacers versus the right spacer to get the right thrust you want? In other words, if the firewall has no thrust built in, then you will need space out the left mounting holes of the engine more than the right to put in right thrust. Thrust Angle Length = Engine mount (distance between mounting holes) * tan (thrust angle) 
To give an example, the distance between the mounting holes on the same engine is 2.75". If you want a 2.5 degree right thrust angle, then: Thrust Angle Length = 2.75" * tan(2.5) = 0.120". The left side of the engine will need to be extended 0.120" farther than the right to have a thrust angle of 2.5 degrees.
So that’s the theory behind the numbers and knowing how good people are with math here is the simple way to do your calculations! I have worked out the Sine and Tan from .5 deg to 3 deg of offsets (see the chart below). Final example: If you have an engine that’s 4.5 inch long (os46 for example) and need 2 deg of right Thrust multiply 4.5” x .034921 (from the chart) = .157 of an inch needed under the left side of the engine mount to give you the right thrust needed!
So the last question, how much is .157 of an inch? Again easy math here with a calculator! In you convert 1/8” to decimal you get .125”. So the next reasonable increment would be 3/16” and that converts to .187 of an inch. So you can see you need to be in the middle of these two points! Finally, 5/32” of an inch converts to .156” ….magic! So simply put, a 5/32” spacer is needed under the left side of the motor mount to finally get you 2 deg of right thrust!
The last word, how do you convert fractions to decimal inches, divide the bottom number into the top number. So to convert 5/32, divide with your calculator 5 divided by 32 to get your .156 decimal inches! Good “well trimmed” flying!!
Rudie