Thanks for all your insights Acer and krshome,

After considering your thoughts I think I will stop at 33" for the head and 26" for the foot end. I had another superb night at those lengths and at this point I don't see a point in going lower as it will fit under my tarp. Obviously I would have a wreck if I kept going and I believe that I have met my objective so what is the point? I was using the 83% of spreader bar factor as my "absolute low as I would go" cut off point.
If I do go to trekking poles at some point I may have to rethink these numbers.

Anyway thanks again, without your info I wouldn't have done anything!