It's very simple: you compare the physical size of the aperture. If they match, the DOF will match for the same framing.
So 90mm/5.6 = 16mm, which would be the same as 50mm f/3.1 (because 50/3.1=16).
For conversion between 6x7 and 35mm, a factor of two is a good approximation; the only issue is that the aspect ratios differ. So 90mm f/5.6 is equivalent to 45mm f/2.8, 110/2.8 = 55/1.4, etc.