QuoteCalcapp just makes the measurement easier, the "30.1mm" puts it to the back of the USB.
I found this number on the SE-76 to be 29.4mm, so if I interpolate the values it comes out practically the same.
This is besides the fact that this all is not rocket science, to quote myself.
Note that I updated the jig dimension measurements above after finding a more exact method.
A note for Ken:
I was not in vain that I invested the work to describe the planes and what to look for.
The "bessey" method I described somewhere here for sharpening an axe uses exactly the same principles. You cannot use any of the angle jigs to set up the grinding angle on an axe that has curved surfaces all over it.