I found the typo in the indicator reading section. I wrote 49.97 which should have been 4.97.
I can't find the error in the probe spacing part.
Printable View
There is a similar gauge used in optics. I think it is called a diopter gauge and it is used to measure the curvature of a lens or a mirror. Nice tool, Tony!
Tony, a typo in the pdf´s Appendix:
¨Firstly we calculate the slope of the two lines AB and BC, then we bisect lines AB and BC with normal lines (shown in red), these will meet at the centre. The slopes of these two lines are obtained by multiplying the slopes of AB and BC by -1¨.
This holds true only for slopes of 1 or -1. Perpendicular lines have slopes that are negative reciprocals of each other.
In the diagram, by inspection, line segment AB has slope of about one half. AB´s red perpendicular bisector has slope about -2.