Hi Andy
This is the reply I got from Bintel here in Sydney:
With the QSI camera the 2" nosepiece can be unscrewed and the adapter on the faceplate of the camera is T-thread.
I have a spacer kit from Orion that has a 5mm, 10mm and 25mm spacer ring.
With these you will have a couple of options:
75mm using the 25mm spacer or
85mm using the 25mm and 10mm spacer
There is also a variable t-ring spacer from Orion:
http://www.bintel.com.au/Astrophotography/Camera-Adapters---T-Rings/Orion-Variable-12-17mm-T-thread-Spacer-Ring/1733/productview.aspx
This could be used to vary the spacing by very small increments if the reducer/flattener spacing is that critical - however it doesn't sound like it if the WO specs give 20mm latitude for spacing.
I have theses spacers on hand so I'll put one of each away for you.
Hope that helps you.
Greg