Draw Tube Travel - how to calculate?

[Stub Mandrel]

I'm struggling to decide how much draw tube travel I shoudl provide on a the scope I am making.

This is thinking out loud to help me decide what to do but other's observations are welcome.

The draw tube needs to be its travel + about 50mm long, I'm worried it could end up too long and cause vignetting.

Apparently the field stop of an eyepiece can be +1.5 -1 inch from the shoulder on the EP so the maximum extension would be 32mm past the focal plane.

A DSLR + field flattener will use up almost exactly 100mm from shoulder to focal plane (55mm camera + adapter, 43.5mm body of flattener) - apparently the non-reducing flattener doesn't affect focal length. This is more than enough to allow for fitting a diagonal.

Add 5mm extra travel inward, just in case.

This gives me travel of 100 + 32 + 5 = 137mm.

Add the 50mm for the part still inserted at maximum extension and that's 190mm which seems awfully long (my cheap frac has a 180mm drawtube, but my newts have much shorter tubes.)

I have a 2" extension and a 2" - 1" adaptor. The 1" adaptor is the shorter at 38mm (2" is 48mm), so I could go for a 150mm drawtube with 100mm travel.

If the end of the 150mm long 1`0mmm travel drawtube is 10mm inside the focal plane,  this would give me:

With FF and DSLR - focal point 10mm in front of focal plane to 90mm behind it. (i.e. should be OK)

With 2" EP, EP shoulder 110mm in front of focal plane to 10mm in front of it. (i.e. not focusing with most EPs)

With 1" adaptor EP shoulder 72mm in front of focal plane to 28mm behind it. (i.e. should be OK)

With 2" adaptor EP shoulder 62mm in front of focal plane to 38mm behind it. (i.e. should be OK)

With 1" adaptor + 90mm diagonal  EP shoulder 18mm behind focal plane minimum (not OK!)

With 1" adaptor and 62mm diagonal EP shoulder 10mm in front of focal plane to 90mm behind it (is that enough in front?)

A moonlight has 4 1/2" travel (125mm) which seems WAY over the top.