There is no ideal return pressure really. You want at least enough flow to keep your sump from having areas of standing water and also enough to feed your reactors, media, and/or algae. If this mj400 is doing that, then I suppose it will work, but I feel like that sump is going to be barely moving.
In other words, if you decide on a skimmer that has a 300 gph
pump on it, you want a return that's at least 300 gph
after head loss in order for it to work at full efficiency.