I have the 118" rail for use with my TS55. Either saw can benefit to some degree from the 118" rail over the 106". To make a full-length cut (TS55) without plunging into the material, I think the rail needs to extend about 10-3/4" beyond the leading edge and around 3" beyond the end of the cut -- thus, needing a guide rail about 14" longer than the intended cut. When swinging from the bracket on an MFT, additional length is welcome to allow room for the fence, if needed. Here, an additional 6 or 7 inches works well for me.
The TS75 will require somewhat more length, as the base is longer. With either saw, the limiting factor is when the gibs reach either end -- poorly stated, but the idea is to have the rail extend before the cut enough for the saw to plunge freely with the back gib on the rail, and at the end of the cut there needs to be enough rail to complete the cut with the leading gib still intact.
Or, you will need to start by taking the plunge...
Sound about right?
Corwin