Sizing an indirect HW tank

I'm going to have my boiler replaced and a new indirect HW tank installed. In my bathroom remodel, I had an air tub installed (takes about 45 gallons if you fill it up max, don't do that though), and the existing Trianco Heatmaker II just doesn't cut it. I've looked at the first hour draw, and even a very small tank provides over 100 gallons. Note in the winter, my incoming water has approached 34-35 degrees after a really cold spell in the winter, so that would derate them some.

The tub supply line is 1/2", so that limits somewhat the tub fill rate, if that makes a difference.

I was thinking about a SuperStor, and the Weil-McLain Ultra 80. My heat loading only requires about 30Kbtu, and the way the Ultra boiler works is if the HW is on, it disables the heat circulator, so all the heat is available for the reheating of the water.

What size tank do you suggest, and, while I don't need it for heat, since the Ultra has a variable rate boiler, I probably might bump up to the next size and not impact the overall efficiency much, would that be a good idea, too?

I know someone knows the answer to this!

The connundrum is how quickly does the tub fill vs the dilution of the tank with incoming water, vs the lowest acceptable outlet temp from the tank vs how much heat the boiler can put back in. If a 40 gal (spec, rarely holds that much in an indirect) tank has a first hour draw rating of 140 gallons, but your tub filler valve can use say 40 gallons in maybe 5-6-minutes, will you run out of hot water? How do you figure out what size tank you need? It would seem logical that if you were using say a 2.5gpm showerhead, you could basically run for an hour and still have hot water, but (I'm guessing here) the tub filler can draw more like 6-7 gallons per minute with a 1/2" line, tempered some with a little cold thrown in so maybe the total volume/min is a little higher.

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Jim you have 360 posts on this site and I ony have 307. I should be asking you questions. This one is a little more specific so you may need to give some time. All the multiple syllable words are more in hj's area anyway.

Most of my posts are repeats of (mostly) common sense. This one is very specific and while I may have the concept down, I don't have the practical experience (nor have I been able to find a good reference source) to make a good decision - in this case I don't want to just go with my gut, since I may end up paying for it for a long time! Just looking for some help. Thanks...