The negative carrier size is 9.2mm. The projection of the carrier outline was measured, this contributes somewhat to the error as the carrier is 3 dimensional and the lens was focused on the negative in the middle of the carrier.

Going from a negative size of 9.2mm to the following sizes, the exposure factor correction in stops is:

Size......Magnification...Metered...Formula...DA Ruler

11mm ...1.2

21mm ...2.3...............1.1..........1.1.........1.3

32mm ...3.4...............2.0..........2.0.........2.0

45mm ...4.9...............2.8..........2.9.........2.8

The ruler and the formula should provide identical results as the ruler is based on the same optical formula - I am not sure where the discrepancy arrises at the 21mm image size. Apart from that they all seem to agree within experimental and rounding error.