IIRC, I ran 26/28 on my 205/40-16 & 235/40-17 R1R. Not convinced it is optimum, but it was not bad.
On my 15/17 setup, I'm at 26/29. Again, not convinced it is optimum, but it is certainly in the ballpark for my car. Rollover looks fine - could even go lower - feel is good, shrug, yeah.
Next event, I'm going to try 25/27, see what it does. We started at 26/28, car was loose, so we let them grow. Not AWFUL loose, but looser than we wanted. Letting them grow to 29 seemed to help, FWIW.
I'm still at the beginning of figuring it out, though, so who knows.