as with all rules of thumb - any short answer would be ineffecient. the "rule" is about 20 but there are a lot of variables, number one being the critters being kept.
deciding on a theme first - then equipment will save you time, money and frustration in the long run
first simple answer: if i were planning a standard mixed reef with zoas, shrooms, and maybe a LPS or two i would shoot for about 20x of sporatic flow
in other words a 200gph HOB and two maxi 900's on a wave timer
second answer: it is my personal opinion that it is really hard to have too much flow in a tank, but very easy to have improper or misdirected flow.
2 other examples:
in a twenty you could easily suppliment your HOB/skimmer/refuge/sump or what ever other periferals you have with a couple high flow pumps like seio 620's moving water behind the rock. a third would be to eliminate the powerheads all together and do a closed loop with a larger pump and spraybars or a scwd
as you can see just with these three options your talking 460gph with the maxis (and half that on a wave timer cause only one runs at a time) or 1240 with the seios and say 500 with a mag 5, all three would be fine, if done correctly but very different in overall flow rate