Both of my E5's came with booms measuring approximately 640mm long. The TT's measured 632.5mm long.
Assuming you fix the boom flush with the front boom clamp, and insert the boom all the way into the tail case, these were my findings:
1) With the tail transmission all the way forward, I needed the TT to be 8mm shorter than the boom for the TT ends to fit tight on both ends. So I needed to cut the TT 0.5mm shorter to 632mm.
2) To minimize unwanted vibrations, it is often recommended that TT don't fit tightly on both ends. My guess is that this could cause bowing of the TT. So, I cut the TT an additional 2mm to 3mm shorter to make the TT between 10mm to 11mm shorter than the boom.
3) On the E5 with with the 640mm boom, the I cut the TT down to 630mm (10mm shorter than the boom). I have the E5 set up with the 4.0:1 transmission all the way forward.
4) On the E5s, I cut the E6 boom down to 700mm. Since I am using the 4.5:1 transmission all the way aft, I cut the E6 TT 11mm shorter, so 689mm.
5) The TT ends have 15mm long flat contact spots, so they are long enough to allow for error in cutting the TT too short relative to the boom.
