The BBB Method, as describe here and at PoolSolutions, is not immediately applicable to a standalone spa, which has a HUGE bather load per gallon and usually is not exposed to constant sun. Spas aren't really my thing, so I'm going to ask one of the contributors here, who's also a PoolSpaForum moderator, if he wants to take a look. But you're welcome to post there, though is a very different community than here.

Best wishes.