Finally I did decided to use round 3/4x 0.035 for my own chainstays and deda stays for my girlfriend. I'll do some deflection tests on the rear end once they're done.
What made me choose the 3/4 over the 7/8 is that deflection of a 17'' long, 3/4x0.035 steel tube with a 50 pounds force is 0.54'' and the deflection for titanium 7/8x0.035 is 0.62''.
I know that bike tubes are subjected to more than just bending but I think it is a good indication. If TI with 7/8 rides well, steel with 3/4 should be good enough.
And I use a 5.5 CLR die from JD2 for my 3/4 and my 7/8 too.