Well, I made the experience that the exact starting point, speaking in millimeters, is not really important.
The camera, in my case a second generation Pentax 6x7 with mirror lock up, auto senses the starting point of the film, sometimes it is further to the start of the film strip,
sometimes it is slightly further back and I have barely any room left on the film strip.
There has never been the case that the last picture touched the end of the film, I always had some room to fit a film clamp.
I've attached two pictures of the inside of my camera, does yours look different? Which version do you have?
By the by, I bought my smc 4/55 from Japan, it was in perfect condition, looks unused an the seller also threw in express shipping free of charge.
That was my second purchase from Japan without any problems.