Help Sizing A Multi-Zone Mini Split System - Minimum Capacity?

Yes.

Now, when it comes to sizing I'd be looking at your average annual low, which is -1F, and has a 27% higher heating load as 12F, which is the design temp I used, so you can go as much as 27% higher without really oversizing. You said earlier your designer was using a design temp of 6F, which accounts for some of the difference.

You try to get the heat loads as close as you can, and then you look at equipment. Typically it comes in half ton increments anyway.

@JustinS : "Awesome… you've given me a lot to think about here, in particular the idea that I should be focus on the heat capacity at the low end rather than the nameplate capacity."

If you take away only one thing, that would be it.

"Although, I think that to some extent I do want to be mindful of the capacity at 47F? Avoiding short cycles and such, right?"

In the spreadsheet I put in a column for short-cycling loss, it's just a guess but if you don't put in something the numbers lead you toward dramatic oversizing. There's a tradeoff, as you go smaller you start coming up short on the cold end and have to supplement with resistance heat, as you go bigger the oversizing losses start to add up.

As a practical matter units come in fixed sizes, usually in half ton increments. So it boils down to picking between two sizes.

You'd have to do a spreadsheet for each heat pump. One of the keys is calculating the COP and output at each temperature, that's going to be specific to each heat pump.