I've been running The Main Salmon - Corn Creek to Carey Creek - for about 10 years now, June - September. August is great. We're launching Aug. 12th this year. It can be pretty hot, but that just makes the swimming at camp more fun. Make sure you take a tarp for shade in camp. Flows will be lower, but the Main is never too low. There will be more rocks & holes to deal, but you won't get stuck every 5 seconds like a late season Middle Fork trip. I'd suggest 14 - 16 ft. rafts & cats. The chute at Salmon Falls can get a little tight at low water for bigger boats. You'll have a blast. The camps / beaches are awesome. Make sure you stop at Buckskin Bill's for water / ice / ice cream. Mackay Bar also has ice & really good milkshakes.
The camp system is a partial reserved camp system, with reserved camps for small groups (1-11 people) and large groups (12 +), and non-reserved camps. You aren't allowed to do layovers in reserved sites. Which way you go (reserve, non-reserve, or a mix) depends on your knowledge of the river, and your preference for "a sure thing" vs "winging it." We usually have a smaller group (under 12) and know the river well enough to go non-reserve the whole way, so that we can do a layover day. PLEASE remember that non-reserve camps are first come first serve. Have a back-up plan is case someone else is in the camp you were planning on, and PLEASE don't whine.
As for daily mileage, that's kind of a personal preference thing, and depends on what camps you're staying at, and layover days. We try to keep it under 20 miles in a day. The stretch of river from Five Mile bar to around Warren Creek is called "Salmon Lake" because the flow slows down considerably, and you can always get afternoon up canyon winds.
Hope this info helps. The main Salmon is my all-time favorite trip.
Have fun & stay safe