There are many ways to get there! The Worldwide Pinhole Photography Day folks have a resource page on line that links to many sources of information. Off the top of my head, 0.9 mm sounds like a rather large hole for a 40mm focal length, but one can over-analyze this stuff too! I have the good fortune of owning a small 50x microscope with a direct measuring scale in an internal reticle. With patience I can measure to about 0.001 inch with that. I believe somewhere in those links is an article that goes into making a pinhole and measuring it with a scanner.

A common do-it-yourself method for a pinhole is to press a small dimple into a sheet of thin brass shim stock and then sand the dimple with fine wet-or-dry sandpaper, used wet. This results in a pinhole with a desirable knife edge. I have made several that way. They may not quite live up to the commercial laser cut items, but they work.

It's fun stuff and there's almost no limit to the possibilities.