I purchased a Spherometer from Thomas Janstrom who lives in Australia. This Spherometer is worth the money and I love it! Very well made and Thomas is a great guy. Here is an online calculator I made that will calculate the ROC, Focal Ratio, and Focal Length of a mirror using his Metric Version Spherometer. Please note that you need to zero the spherometer with the supplied glass flat and then subtract the measured sagitta on the spheremeter by the zero point.

For example, on my spherometer, zero is at .64mm. Measured sagitta is 1.10mm.
sagitta = 1.10 - .64 = .46mm

This calculator only measures concave surfaces

Please contact me if you have any questions or comments.