[mch62 (Mark)]

Have you used Newt 2.5 a free ware program.
If not get a hold of it and first read the help files in particular the section "notes on scope design".
Punch in you numbers and work from there.
There is a better program around but this one is quite easy to use.

From what you have given your obstruction ratio is about 20% , visualy any thing under 20% you would be hard pressed seeing the difference unless really experienced at seeing details.
The important thing is the 100% fully illuminated field and yours works out at arount 12mm.
You want to have that at the size of a any CCD chip your going to use and or around at least the 75% illumination to cover the size of the lowest power eye piece field lens(the bottom lens).

your 75% is about 27mm.

If your really wanting to go smaller in secondary size you would need to use a low profile focuser to maintain a resonable field size. like 25mm profile.
Also using a smaller tube diameter but this will cause heat currents in the optical path unless you use an open truss tube.

So it's gets to be quite a change if modifing an existing OTA but feasable if starting from scrartch to make a ultra high contrast Newt while maintaining a resonable fully illuminated field.
In your case it's not worth the small change in obstrction ratio for the expense.

Mark