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I would consider leaving at least 1 mm of clearance. In the original model, there is 24.5mm for the rod to fit in, so it seems I left about 1.08mm of clearance, which has worked well for my purposes.

How much to leave is really dependent on how tight a fit you want, and the tolerance that your printer can achieve. My printer tends to bleed over about 0.1mm or so over the tolerance of the model, so I would add at least 0.2mm to try for an exact (very tight) fit.

When computing your scaling factor, I'd use the dimensions which include the tolerance, so 24.5mm for the original instead of 23.42mm, and 20mm instead of 19.5mm. So, you'd get a scaling factor of 20mm / 24.5mm = 81.63%

One other thing to take into account when scaling the model is that all of the features of the model will scale proportionately. That includes the holes for the M3 bolts and the cutouts for the M3 nuts to nest into. Unfortunately, those will be too small after scaling to function as intended. The holes could be drilled out larger easily enough, but the nut cutouts would require a bit more work to address.