Yesterday I tried the scotch tape method on the Bessa I 6x9 and the image was clear and easy to see. However, the results varied wildly, both longer and shorter than measured distances. There were even differences on multiple measurements from the same position! It came down to 3 mistakes.
1) I applied the tape across the 9cm opening, 2) the frame members at either end are about 2mm closer to the lens than are the transport pins and the upper and lower members that determine the actual film plane and sometimes the tape stuck to that, and 3) tape can curl, skewing the results and 9cm made that to easy to happen.
Next time the tape will go vertically across the 6cm dimension and will be taped to the frame that guides the edges of the film.