I am going to flush them, what should runoff b 6.0? And i want to resolve this so it doesnt happen again.
I never had salt buildup problems until my runoff was lower than 5.4 and the ppm was in the high 1200's. Without seeing the whole plant and more pictures in natural light, you might get by just feeding water only and reducing your nute strength from then on. If you flush, I don't believe there's a ph or ppm you're looking for. You just do a 3x flush. I've never flushed. Sometimes when I fed water only I watered with 2gals for 50-100% runoff. A moderate flush. But, never a 7.5-gal flush (for my 2.5gal containers).
Something to keep in mind about runoff ph: This isn't accurate except as a trend to watch (which you've been doing). How long it sits in the container before being displaced into runoff, and how much water is added to displace runoff (potentially diluting the water which sat in the container) affects the ph of the runoff. Like I said, as a trend of your watering style, I think it's valuable information to have when changing nutes or medium. But, when you flush, it's obviously going to be a different style than what you normally do. (And, you can't compare your individual runoff values to mine. All we can do is compare trends such as the velocity toward acidification.).
Google for NCSU Pour-Thru Method to see the most comparable runoff ph. Few people have the time to wait an hour. So, the best we can hope for is fairly repeatable watering style and a sense of how each watering was different (how today's 5.5 might reflect differently than last week's because we were in a hurry this time, or got more runoff, etc.).
You said you want to avoid this in the future. If it's not salt buildup from overfeeding, you'll want to amend (more?) dolomite lime into the soil. Maybe also begin
dissolving egg shells in vinegar (<<link). That will give you a source of ca to treat a deficiency without adding unnecessary mg (via a calmag product).