Strategies for measuring physical PCB dimensions
I like your optical comparison method; what you need is a small microscope for your cameraphone to plug into, that would work.

Foldscopes come in at 5€ and do that (German shop, sure there's shops outside of Europe, too), since you don't need high magnification.

Also, replace your caliper by a nice thin sheet metal  ruler; you can make one yourself or get one in the shape of square pins that come in pretty well-defined widths; I'd probably do something like getting a nice flat, thin piece of sheet metal, like a razor blade, cutting it down and then filing it until it's of some defined width, according to your caliper (0.3 mm sounds doable by hand, if you support it from below while filing), and then holding it flat onto my trace to be measured – that eradicates the need for the camera optics to have a large focus depth.