Thank you everyone for your responses!
Anyway, I like the BTU calculator but I do wonder if having a hull sitting in cold water alters the variables a bit. On the other hand, isn't unfrozen subsurface water usually warmer than surface air temperatures in winter? Maybe iceaters and bubblers actually affect boat temperature? So much to consider!
Seems like we need a calculator specifically for marine use, taking into consideration more varied weather regions. Until then, experience is our guide.
We have a Webasto Airtop 2000 on a 33-footer in Maryland (that's faaar south to you) that we haven't had to turn up more than halfway over the last winter. Our boat doesn't have a headliner, either. Webasto's website had a primitive calculator to help size the product to your boat and location.