It is common to use a small dial calculator to translate the smallest commercial light meter reading (typically f/32 to f/90, depending on the hardware) into the f/256 to f/512 range common with pinhole. The trickly part is to also incorporate the effect of reciprocity failure into the exposure. I have seen home mnade dial calculators that have two sides; one for the smaller aperture translation, and the second to do reciprocity work arounds.