This is only my personal experience and some people won't like it but here goes -
I had two of these fixtures on low tech (no real fert regimen, no CO2 or excel dosing) and I got bad algae on the surface of everything. Plant leaves, rocks, etc.
What I ended up doing (because I am lazy and because I was more worried about my shrimp than about plant growth) is that I tossed in enough floaters (frogbit being my favorite) to cover a good portion of the top of the tank.
This probably cut the visual spectrum of light hitting the substrate in half. I never bothered to measure PAR.
Between the floaters blocking light from the tank and the floaters sucking up nutrients like crazy (and not needing much CO2 since they have access to atmosphere) my algae problems went away pretty quickly and never came back.
The downside to this method is that it can make getting good growth of rooted plants difficult since the floating plants outcompete substrate level plants so easily in this situation.
There is probably a way to manage a balance of sorts but I never bothered to try.