It seems to me that these can't both be correct? If I understand correctly,
@wiltw would put both the sensor-to-lens distance and the subject-to-lens distance at (50mm x2 =) 100mm for each distance, symetrical around the lens node, right?
But it is probably a moot point, because as already mentioned, the OP is copying 35mm to APS-C, so is probably not using a magnification of 1:1, but more likely about 0.65x, which is the magnification needed to fill (many/most?) APS-C sensors with the image of a 35mm negative.
However, if taking multiple overlapping shots by filling the width of the sensor with the height of the film, then the magnification will need to be very close to 1:1. (My Fuji APS-C sensor is 23.9mm wide and a 35mm frame is 24mm tall.)