I believe all you'll need to know, is what the exact Spindle-Pivot Distance is?
Once you know that specification, then use the most accurate means of measure to insure you drill the Amrboard-Arm Mount to that specification.
Remember, that measurement must be taken from the exact center of the Spindle, not the side.
It won't matter where exactly fore-aft the hole is drilled on the Armboard, as long as that specification is met.
Also keep in mind, when taking such measurements with any kind of ruler-caliper, or measuring device, to insure that measuring device is perfectly level.
To do otherwise, and have the ruler-caliper slope down, you will not get the exact distance you desire, and can possibly be longer than the spec'd distance. Mark