Calculating Heat Loss in Open Tank

It’s been a very long time since I’ve done any technical calculations, and I’m a bit stuck on a seemingly very simple task: calculating the convective heat loss of a heated bath.

My biggest problem is finding a convective heat transfer coefficient to use. Engineering toolbox gives me “typical values for free air convection” of 0.5 to 1000 W/m2K.

How do I size a heater when the heating requirements range by 2000x?

Does anyone have a better source for coefficients?