My experience is that about 35 CFM is normally enough for a house with two or three people in it. The house is fairly tight at 0.85 ACH50. We run the HRV at a higher speed for a couple days when the weather first gets cold to control condensation on the windows.
Don't forget freeze protection. In cold weather, moisture in the outgoing air will freeze inside the heat exchanger and plug it off. My (purchased) HRV has a frost removal cycle that turns off the makeup air blower, and switches the exhaust air blower to high for one or two minutes every half hour or so when the outside air temperature is below about 20 deg F.
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