Alistair,
go to k3CCD (
http://www.pk3.org/Astro/) and download lite version of K3CCD tools (it is free, however requires a key).
Software is here:
http://www.pk3.org/Astro/Downloads/k..._1_1_7_541.zip
Key is here:
http://www.pk3.org/Astro/k3ccdtools_freekey.htm
This will give you the ability to precisely measure PE. Of course, longer the FL, the better. Also, this software can be used for guiding (you may need it, considering your gearbox).
Orientation of the camera is not critical, but it should be such that the drift (no tracking) produces trails parallel to X or Y axis of the frame.