The combined flow (Middle + Main) at Shoup right now is roughly 3200. That is below 0 feet at Corn Creek. 1 foot at Corn is about 5,600 cfs. 0 is around 3,400 cfs.
Not easy to find but there is a hydrograph at the Salmon-Challis National Forest website that does what you are asking. I just had it but lost the link. I saved it in PDF but unable to attach. I ignore Whitebird because the flows there are always greater than they are Corn Creek, with all the tributary streams entering, etc. (including South Fork, Little Salmon at Riggins, and all the creeks). To run the Main the best info to rely on is what the flow is at Corn Creek.
I wouldn't be too worried. 2-3K, even less, is enough to float the Main. Just expect it to be low. It's not going to change much between now and September. There will be plenty of beaches. Lowest I've gone is about 1 but I felt there was plenty of water and it would have to get a lot lower before it would cut-off boating.