Just a bit of a warning about getting caught up in the fractions of a millimetre for these adapters etc. I went down that path, calculating backfocus distances, light paths and so on, taking into account manufacturer's tolerances of the camera - worked out 19mm extension tube (give or take a couple of mm depending on the fabrication tolerances) was needed. Stars weren't round. Did some testing with various length extension tubes, and found the actual required distance was 30mm! Over 10mm more than the theoretical calculated distance.
What I'm trying to suggest is that trial and error and testing of the distances first, rather than calculating, before spending money on Precise Parts fixed length extensions would be my recommendation.
I found a handy variable extension tube from Stellarvue that in combination with their other ext tubes allows me to test a wide variety of spacings before buying/making a fixed distance one.
http://www.stellarvue.com/42-mm-extensions/
PS - don't bother with the Orion variable like this one:
http://www.amazon.com/Orion-12-17mm-.../dp/B00I44ETRE
It has nylon grub screws that are very easy to damage, don't hold weight well, and just too much slop in the mechanism.