The current BSA breech extrusion does measure 13mm across the widest point of the dovetail rail. to see how well suited your mounts are to positioning the scope centrally put one mount on with the screws on the right, the other with the screws on the left with both mounts together, a misalignment of the mounts shows that the scope will not be mounted centrally when mounted with both mounts the same way round.
BKL mounts are a good choice as they put your scope dead centre. The offset with mounts made to suit a narrow rail will be small and of little impact on the setup but every little helps!
I also see that BKL now do a lower 30mm mount, well they call it a low, it gives a scope centre height of about 1.65" on a BSA.
HFT, for people with only 2 buttucks!
VooDoo TwoTwo The next chapter.