(can't resist) "Hello, Bruce" (see if you can tell what movie that's from)

1/4-1/3 of the bottom should be the amount taken by a UTH. That way you have 1/3 warm, 1/3 'neutral', and 1/3 cool.

Also, tanks can work just as well as tubs. It all depends on the owners desires, the room environment, and the insulation of the tank.