Firstly, my developer of choice is D76 @ 1+1
Secondly, I don't think the rotation rate on the CPE2 will make that much difference, although there should be a measurable difference, but I don't have anything to measure it with.
The two speeds on the CPE2 were, as I understand, designed to give the same relative speed for the two sized tanks the unit is capable of handling. In other words the 1500 series of tanks rotate at the faster speed and the larger diameter 2800 series rotate at the slower speed. The relative outer speed of each tank, should then be about the same.
The handbook I have, doesn't mention speed anywhere in the book, that is just my assumption. That said, I have always developed at the slow speed for all films, whether in the 1500 or 2800 tanks. When I developed paper in the 2800 tanks I always used the fast speed.
Having developed C41, E6, B&W reversal and all manner of B&W film and developer combinations in the two sizes of tanks the unit can handle, I have never had evenness problems with the slower speed, except as mentioned previously.
I would suggest that you mix the amount of developer required to cover the film, plus a safety margin, eg:- 270ml required, use 300ml, then just develop your films.
One of the reasons I don't like HC-110 is the extremely small amount of concentrate required. The smaller the concentrate required, the easier it is to get slight differences. If you are over or under .5 of a ml in your 4ml measurement, then you can be out 12.5% in concentrate, that is a substantial and presumably measurable on the film difference.
Mick.