The proper way to measure stamps is simple.
Either take out a ruler, some calipers or whatever you like best for taking measurement.
Find the stamp you are looking to measure and try and measure it.
Mare sure when measuring you measure from the inside of the stamp so from the center of the line to the center of the line.
Here is a sample picture to help explain.
If you measure from top to bottom you can cause a miss read of the stamp and order the wrong stamp for your Resto.
If you have any questions on this topic please feel free to email me