How big can I go while keeping the same light?

[OCDP]

Well, I mine as well start preparing and getting info. together for my new tank. I am not in a major rush to get it going, but I'd like it up within the next 3-4 months or so. I don't want to upgrdae too big, because I just don't have the room, but anything is better than the current 20g setup.

I want a cube, I know that much... but I wanted to know, how deep could I go and what general size cube (in gallons) could I go while still keeping only 1x150w HQI over the cube? I do not want to upgrade lights.. I don't want to have to spend too much, I am doing this mainly for the fish, for more room to swim, and more room for corals as well.

I do keep SPS , all acros basically. Thanks in advance , I am looking forward to planning out this setup. It will be basic.. just as my 20g, sumpless and no refugium.. I may make an AC500 fuge though.