I would start with about a gallon of fresh. You really only need the salinity to come down a bit. For a 40 gal
I would think a gallon would bring it down a point or two. When did you take the reading? PH lowers at night and starts to raise when the lights come on. What I see in the picture looks to be just about right. Especially if you took the sample early in the day. I prefer not to dose anything if I can help it. Let the tank find its own happy medium unles it gets way out of line.