What I did was do a full ray trace on paper. Step one was mark on the cone my Desired Field Size....mine is actually 37mm not 38 but no difference really. Then I followed the instructions with lines going all over the paper, this part was confusing to read, but easy in practice
Then I had where to place baffles. On my rather messy drawing I had already guessed where to put baffles, then found the Berfield method, which gave me less baffles and put them in the right place. The only one I didn't do was no 1. I found from Refractor builders on Cloudy Nights that flocking is as good as a baffle so I did that. Looking down my tube in the Daylight all I see is ...nothing..... just Black.
How long to make the tube is variable. I wanted to use WO Bino's without a barlow/OCS so I took the 110mm of those + 2" diagonal and a GSO Linear Bearing Focuser into account. My tube lenth is 1430mm roughly. In the pic no2 it says 1530 but that was my pre Berfield guess... miles out, 100mm to be exact. Easier to draw than cut an aluminium tube in the wrong place.
It helped to put the objective on a length of wood and have an eyepiece in the focuser on a sliding wooden job, and measuring that. I forgot about close objects focusing further out than infinity objects, about 10mm in it.
I also asked Ales the owner of Istar what they do for tube length and the answer was take 300mm off the objective FL....mine is a bit shorter for binoviewing native.
Your tube might end up at 1200mm
Some photos 'might' help....