Michael, measure from the lens' diaphragm, not from the front, whatever that means. This because most LF lenses' nodes are close to the diaphragm. The front node, from which lens-to-subject distance is calculated, will be a little in front of the diaphragm. The rear node will be a little behind the diaphragm.
I take it that you used a 300 mm (12") +/- lens. Since distances given magnification are calculated in focal lengths, doubling the focal length will double all distances, holding magnification constant.
About measurements and such. The closest the rear node can be to the film plane is one focal length. The subject will then be at infinity. This is, in fact, one definition of focal length. The closest the front node can be to the film plane is one focal length. The film plane will then be infinitely far from the rear node. Your measurements are a bit, um, sloppy.
About good practice. Lenses intended for general purpose use, such as the ungodly expensive XXL 1100 are optimized for a big subject in front and a small (relatively) negative behind. When used at magnifications > 1:1, there will be a small (relatively) subject in front and a large negative behind. To get the most out of the lens' optimizations when working above 1:1 the lens reversed.
To be fair, the XL 1100 is a dialyte and might be nearly or perfectly symmetrical. Quick check, compare the two cells' focal lengths. Equal means symmetrical, doesn't have to be reversed for > 1:1.
There's a big exception. Dialyte type process lenses, e.g., Apo-Artars, symmetrical type Apo-Nikkors, Apo-Ronars and Repro-Clarons, are perfectly symmetrical and don't have to be reversed when used above 1:1. Plasmat- and Dagor-type process lenses are also perfectly symmetrical but the likely ones, G-Clarons, are too short for your application. Apo-Tessars and clones are asymmetrical and should be reversed when used above 1:1.
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