I have only used flexwatt with tubs and now I have a RBI rack with 4x 32qt tubs.
When I got my first BP I had a 10g tank with UTH and a lamp. I never combined flexwatt with the tank so Im not sure, but it seems logical to use it under. I would set it up around 94-96.
If I were to use a tank again I would probably cover the whole under side with flexwatt split in half, with each half controlled at different temps so I wouldnt have to use a lamp again. I would probably do the same thing with a cage.