In the old days, before exposure compensating macro lenses and autoexposure flashes, I went through the math of flash macro photography. Over a modest macro range the bellows factor and the square law light fall-off compensate for each other. I don't remember the details, but the flash was an anemic 1960s model, and the indicated aperture on the lens was f/22 for Kodachrome 25 over a macro range typical for photographing flowers. The flash was mounted fairly far from the lens for better molding of the subject.