Return pump?

[ClownieandBilly]

I intend having around 15-20 x tank volume per hour circulation on my new 120g reef set up.I am going to have a 25g sump under my main tank to house my heaters,skimmer and return pump initially and any other equipment i find neccesary in the future.What flow rate do you advise my return pump to be able to produce?I will use powerheads to aid my flow rate to acheive my 15-20 x tank volume circulation