in a recent post, that I can’t find anymore 
@bar asked @dlang ( and me I think) if it was possible to introduce a gradient into the frame size checker, so not a hard line where one side is accurate, and on the other side it is inaccurate. This got me thinking, because of course by passing the 130 degree arc in the checker (moving to a corner of the frame), the belts will not be a straight line any longer, and there will be a fault in the measurements of the machine. But how much? At first the angle between the arm and the belt will be small, but the further out to the corner, that angle will increase, and so will the distance between where the machine thinks it is and the place where it realy is.
So I drew up in CAD a standard frame of 3048x2440mm, and a workpiece of 2440x1220mm. On the diagonal of the workpiece I constructed the 130 degree point from the framesize checker, I called that point zero. Then I constructed other points on that diagonal between zero and the corner of the workpiece, with incremental increases of the angle between arm and belt. That angle was about 9.5 degrees by the time I reached the corner of the workpiece. On every incremental station I constructed the true position and the position where the machine would think it was, measured the distance between the two and put the results in this table:
on the vertical axis is the difference in position of where the machine thinks it is, and where it actually is, in mm
on the horizontal axis are the distances from point zero outward, also in mm.
So here we can see the “gradient”: 160mm out of the 130 degree arc of the checker, the deviation is about half a mm (that would not concern me)
400mm out of the 130 degree arc of the checker, near the corner of the workpiece, the deviation is more than 2.5mm (which would concern me 
)
So in this example, I think it is a good idea to keep the calibration area inside the 130 degree arcs of the checker ( and even higher, if you don’t want the effect of rotation of the sled), but for actually routing, you could easily extend that area to the 122 degree arcs (you can adjust these in the checker, and observe the larger area) and be only half a mil off.
In Theory that is 
Arjen