The set back deal is mainly for the front discharge blowers. They have an outlet that is a small delta (pie shape).This pushs the air all to the front of the case and it comes out at near a 45degree angle.

All helixed rotors push the air forward - look at the Eaton. How far forward is affected by the helix angle.
The helix is not the actual rotor angle, but how many degrees the rotor is "clocked" over its entire length. A really short 60° helix like the Eaton M45 has a very wide angle, while the longer M90 with the same 60° helix has a much narrower angle because the distance for the 60° turn is double.
Part of the exhaust masking is to prevent back-flow from the manifold into the rotors in the rear of the case, which just adds temperature and pumping loss.
Where to start? Examine the exhaust ports, and measure the angle of each rotor. As you can see, the high helix "pie slice" will be shorter with wider angles due to the helix. Draw a triangle the same width as the port, and make the sides at the rotor angles.
The range of exhaust length will vary between:
1. placing the point of the triangle at the back of the case (largest), to
2. starting the angled sides at the front of the case (smallest).
Which to use? Depends on fuel, blower vs. engine size, engine speed, etc. - I can't see a pattern yet.
If the pie slice is the same area as a 4500 opening (on a Victor or other good single plane manifold) use that manifold (far better than a conventional 1950's technology flat manifold), and make an adapter plate with the 4500 pattern.
If the pie slice is bigger than the 4500 opening a tunnel ram is a better basis because the top has more room, but the exhaust should still be near the center of the top unless you want to play with diverters etc. inside.