Don't disagree that different calendars can have very different crowd level estimates. and do check multiple calendar estimates myself. But of the "free" calendars, UnderCoverTourists has tended to match my actual park experiences more closely than the others. Though your point to check multiple calendar estimates is a good one.
Tour Plan has a decent one as well. But it requires a subscription to get the full schedule and more recently (i.e last 5 years or so) their actual "number" crowd ranking during the lower crowd level times has tended to underestimate the crowd levels more than UnderCoverTourist's. (At least that has been my experience.)