Using a micrometer to measure by eye (difficult to do) I'm pretty confident that the hole is between 0.25 and 0.3mm. Comparing with a simple ruler at least seems to show that it's significantly smaller than .5mm and it looks at least around 0.3mm on the meter, so I don't believe it's terribly big or at least not more than maybe 25% larger than I was shooting for.
Anyway, I have a 10x loop and a basic magnifying glass (and I'm very nearsighted... one of the few times it comes in handy) so it's not like I can't get some idea.
It's still hard to see the exact regularity of the hole of course, but if I hold it up in front of a lamp in the distance I can see a sort of irregular sunstar that makes it look like I have burrs inside the hole (despite sanding etc). I'm unsure if this means there are really burrs, or if an irregular sunstar is just some artifact of diffraction in tiny holes combined with an imperfect circle shaped hole.
So it's hard to tell but right now I'm going for as close to perfect image sharpness (not resolution) as I can get (which is what mrpinhole calculates, I think). Not surprisingly I didn't get there with crude tools but I'm still curious as to what's most important to improve compared to what I have now.
For now I used a square cut out of the side of a soda can which is maybe twice as thick as cinefoil which is probably around twice as thick as aluminum foil (which I suspect is so thin I'd have problems with it). I also can't really get it perfectly flat. (Perhaps I should have just let it stay bent and tried to drill the hole without deforming it. After all I'm guessing that trying to flatten it might make it more irregular, though I have no idea how the cylendrical bend might cause problems.
So it sounds like maybe what I need to improve (in addition to my tools) is the material thickness and perhaps burrs, or at least get more confidence that I don't have burrs. Maybe I should try the electrical zap method on foil? (I do have some capacitors sitting around, hopefully enough total farads to do it.)
Is half the thickness (such as Cinefoil) good enough or should I get something thinner? I think the coke can is like 0.2mm thick and the cinefoil is 0.1 or something. 0.2 is almost the diameter of the hole I'm going for so I'm guessing maybe that's an issue.
I'm planning to extend this thing out anyway where an 0.5mm hole should supposedly be used so I'm guessing that will make things a little easier.
To make things more complicated, the instant film I'm using doesn't really have the best fidelity anyway, at least in the print. (I think the cleared negative should be better.) "Resolving power" is listed as 15 lines/mm. Don't know if that's achievable with a pinhole this size anyway though, especially if not optimized for resolution.
Anyway, main question now is: Is the material thickness likely to be the biggest flaw in my current pinhole?