The general rule of thumb is you can only get about 75% or so of the content of a WH before you've diluted it enough to dissapoint you. Some of this depends on what temperature you maintain the WH...a tank with 140-degree water will fill more of a tub than one held at 120-degrees. Most indirects are maintained at 140-degrees, and most codes require a tempering valve on the outlet to restrict the output. Some boilers can let you up the storage temp even further, thus making a smaller tank 'look' like a bigger one. The higher the storage temp, the bigger the storage losses, but on a good indirect, it's an order of magnitude less than a gas fired standalone tank. In the scheme of things, making it a priority zone is still probably your best bet. I have a 60-gallon SuperStor Ultra on a 60K boiler, fill a 6' air tub, and have never run out of hot water. Mine is plumbed as a priority zone, and is maintained at 140 with a tempering valve to limit the outlet to 120 or so. It does take about 10-minutes to fill the tub, as the supply is 1/2" pex, but that's another issue.