Very good idea! I am going to go find a microscope.
... Thanks ...
... optical distortions ....
In order for the measurement to be accurate, you need to know that the lens does not distort the ultimate image. Unfortunately, even with the best macro lenses, you will get distortion and the measurements will be off. The more the magnification, the greater the problem will be magnified.
The (partial) solution is simple though. You need to correct the distortion. I suspect that the easiest way to do that is to measure the image of the ruler on the screen. If the measurements of the ruler are accurate, then the rest of the image in the same plane of critical focus as the ruler will also be accurate. Note that the correction will only work for items in the same plane of critical focus as the ruler. Accordingly, I suggest that you use as small an aperture as available, but even then, the ruler and the razor must be as close to one another as possible. For this, an accurate tripod and accurate measurements lens to subject will be crucial.
Gillette Red tip - measured 25.5 thousandths of an inch
Slim Adj set to 7 - measured 36.1 thousandths of an inch
Aristocrat - measured 25.5 thousandths of an inch - exactly the same result as the red tip - which does not surprise me.
some metric conversions: top end en bottem end of scale only + red tip / slim 7
0,015 = 0,381 mm
0,040 = 1,016 mm
0,0255 inch = 0,6477 mm
0,0361 inch = 0,91694 mm
so that the metric folks have an idea.