That completely depends on what you want to keep in your tank, your aquascape and many other things in sure! Tell us a bit about that
Flow is not a set it and forget it thing, you're gonna adjust it a million times as your tank nurtures! Chances are that the MP40s aren't even the right tool for the job- they are versatile though and a good place to start.
Get your new tank, make the scape you want and consider what coral and other lifestock you're gonna keep- and where you want to place it. And then decide on flow!
Personally, I run a 4 foot tank with
2 x MP40
2 x ReefWave 45 gyre
1 x MP10