This fall, I bought my first CCD camera for my Celestron EDGE HD 800 - a ZWO ASI1600MM - Cooled, with their 5 POS EFW (since they didn't have a 8 POS at the time). Since I'm new at astrophotography, I bought their LRGB filters specific for the 1600MM-Cooled.
I had the same questions as you have and came up with the calculations as follows:
With Celestron 0.7x reducer : 105 mm
Celestron T-Adapter: 50mm
Adapter extension (comes with Celestron T-Adapter): 28.5mm
ZWO ASI1600MM-cooled: 6.5 mm
EFW: 10mm
Total back focus: 95mm
I bought a set of different T-ring extensions from Telescope Camera adapters for about $USD30. They are really well made and used a 10mm extension ring to make up the needed space (Total 105mm). Note I have no affiliation with this company but like you were assisted by other astrophotographers.
I've just bought a OAG to assist in eliminating flexure from my setup.
Good Luck with your ZWO camera - it's great!