Can they print precisely enough to create usable rulers?

I’m hoping to achieve millimeter scale.

You should check out the Castable Resin page. There’s a graphic towards the bottom that shows the accuracy of the printer. We printed a model and then took a 3D scan of the print. The scan was then compared to the original design and over 90% of the model falls within 100 microns of the intended dimensions.

You can fine tune each machine to get extremely accurate prints. With the standard resins you will get some variation based on IPA rinse time and post cure. Formlabs has included fine tune features that allow you to tweak in the models effectively.

So, yes it can be done. You do need to be careful about the print orientation to print a scale or any “accurate” part.

As you can see from this image printed in Grey V2 at 100 microns the numbers and lines can be read (black or the grey V3 may be easier to read). The measuring edge came out reasonably straight but the back side (support side) is curved. The file was from thingiverse and I would recommend designing the lines a little deeper and wider.