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@hoefla
no way, setting stuff up and making things easier than optics calculations! - switch off light, face window. lock shutter open if you can, prop up lens. maybe make a quick frame to turn trace into a screen. move tracing paper away from it until distance stuff in focus, check focus with lupe on back of trace. measure distance. repeat with objects closer up as required, shine a desk lamp at them.
anyway, there aren't any sums, if it's a "true" 105mm, I would have thought the distance will be from the middle of the (thickness of the) lens, but the calibration is just to check that. I think.