Your comment about having a carbon filter reminded me of something. Like you, I have a 10" carbon filter to prevent chlorine from damaging the desalination membranes. We also use one when filling our tank with (verified) dock water. I noticed on a sticker that it was rated at 2 gpm. I did some research to discover that these are rated at 1, 2, or 3 gpm depending on what a manufacturer claims. More Googlerian research revealed that chlorine removal is dependent on the length of time water is exposed to the carbon as it flows through. These are not high rates
Anyway, it seems that when filling your tank or flushing the water maker that slower is better....1 gpm is slow. When we have the luxury of time we use this rate to fill out tanks.
We think it is very difficult to control the flush rate by cracking the, 3 way. When flushing our membranes I turn off our water pump and let the accumulator tank run out of pressure. Then I open the flushing valve fully and control flow by short pushes on the water pump breaker. I keep water flowing for the 3 minutes of recommended flush time. You can monitor the flow pressure on your Dessalator panel.
BTW Seagull filters are also rated at just over 1 gpm based on the same constraint. They even sell kits for restricting the gpm to 1.
Bob, KAIMI SM 427