There has been a lot of variance on the advice related to flow. I once thought that higher flow was better, so much so that I deemed my Rev 4 "HF" for High Flow - the slot pipe was locked in place, you could pump a lot of water through it without having to worry about it flying away lol....
Real world feedback revealed that once you get up near about 50 GPH per inch of screen width, you actually end up with a negative result, even if you have a well established screen. Meaning, high flow in fact did not necessarily result in even better growth, but the opposite.
This really doesn't have anything to do with washing the algae off the screen. If you can't get algae to attach in the first place, then yeah high flow is a negative. But when you're talking about a well established screen that then had high flow applied to it, I think what was happening is the flow was primarily bypassing the algae mat (going right over the top of it).
Then I started getting feedback from people who I thought were under-feeding their screen (too little water flow) but their growth was actually pretty good.
The original "guideline" was 35 GPH per inch of screen width. I designed the HF to handle 50 GPH/in or more, but still had recommended the 35 GPH/in minimum.
Around the Rev 4 release timeframe, I started telling people to begin with lower flow - basically just enough to cover 75-90% of the screen with water, without it "arcing" from too much flow. This ends up being more like 20-25 GPH/in of flow, sometimes as low as 15 GPH/in. One reason for this is because it's very hard to get even flow at 35 GPH/in, you almost always get arcing. So this avoids that issue. But also, to get algae started growing on the screen, you don't want to blow it away with a ton of flow - so this made sense, drop the flow down to the minimum for just coverage.
Then, I tell people to increase over time as needed. Being able to adjust the light intensity really helps here, either through physical distance of the light to the screen, diffusion, shading, or dimming. So if you need more growth, generally that means more intense light (less shading, less dimming, etc) and more nutrient delivery (meaning more flow)
But sometimes you don't need to increase flow - that's what I found out. It all depends on your tank. So don't fret with "following the guidelines". Start with good coverage and get growth started. Take it from there