Thanks Jeff. What would I do without you?
I checked a second time without the softener attached, and got 600 ml/min, so that's not so bad. I think the variation may be caused by the power to the pump being less when the heater is on -- could be that the wiring in my kitchen is not entirely up to snuff...
The only annoying thing is that I checked the overflow rate from the return tube into my tank with a blank basket in, and it's only 100 ml/min, which would seem to indicate that my OPV is set too high -- except that with the flow at no pressure varying so much, I guess I can't really rely on that measurement. Guess I'll need to borrow or buy a portafilter with a pressure gauge to be sure.
Thanks again for your help.
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