Thanks Leon - you could be just about spot on. I found this thread which quotes a Tak dealer as saying the spec is +/- 2mm:
http://www.iceinspace.com.au/forum/s...d.php?p=381255
I've also been told that the 2.5mm glass filters extend the desired distance by ~1/3 their thickness and I've shaved another 0.5mm off my measurements so I think I'm now as close as I can be given all the uncertainties.
Phil