Hello: It’s my job to cause trouble and I takes it seriously 😎 A fifty gallon heater will deliver about 75% of its volume, or 37.5 gallons of undiluted hot water. The recovery rate of the heater you’re looking at is 43 gallons per hour. Let’s say you expect to fill the tub in 15 minutes. You would get 37.5 plus about 11 gallons or 48.5 gallons of hot water. No doubt it will need to be mixed down with cold water, but you might not get enough hot to fill the tub in winter when ground water is cooler… and then it might work just fine… but this is why I suggested a tankless heater just for tub filling.
Tub filling really throws off equipment sizing.