Start at the Denny Creek trailhead (which is now at the big parking lot for Franklin Falls off the NF-5800), up the trail to Lake Melakway, continue on up to Upper Lake Melakwa to Melakwa Pass, then descend to Chair Peak Lake and then traverse (mostly off trail, though there is a faint boot path off and on) around to the ridge below Roosevelt Peak that traverses to Gem. From Gem follow the normal trail back down to Snow Lake and then up the ridge and down to Alpental. About 12 miles TH-to-TH.
A variation is to close the loop by connecting the 2 THs on the roads. About 16.5 miles total.
I attempted this route today and missed the FKT by less than a minute. I gained good time up to Melakwa Lake but snow coverage was extreme, packed and hard. Steep traverses were dangerous and I really couldn’t move across the snow with any efficiency. I’ve got grippy shoes but no spikes. I lost a lot of time from Chair Peak Lake, across Gem and down to Snow Lake. I was hoping for a fast run the last few miles but snow persisted past Snow Lake and the rough rocky trail is a slow jog. I passed 161 people or dogs on the loop. 150 were from Denny Creek campground up to Lake Melakwa. The crowds actually didn’t slow me down since people are afraid the COVID monster will instantly kill them if I pop their 6ft bubble. Almost everyone stepped off the trail quickly. I was the only idiot traversing the top lakes. I intend to try the loop again at the end of July with less snow coverage. I think it was a bit dangerous at this time without spikes and an ice axe thingy. I still improved the full car-car route in 5hr:09m.
I would love to hear a report after your second attempt! I plan to set the female record this August, would love to know if ice axe is needed.
My report is detailed in my successful FKT on the TH-TH on 7-18-2020. I did utilize my YakTrak snow treads and hiking poles because there were a few steep lateral traverses and a climb up from Malakwa Lake over the pass to Chair Peak Lake. The snow had melted considerably from July 3 to July 18. I am fairly certain you will have virtually zero snow at any time in August. You will certainly not need any snow gear. There are a few steep grades so just have good tread on your shoes. There is either a large trail or a well worn path the whole route so it will be easy to follow now that it's dry. Once you reach the saddle and ridge traverse from Chair Peak Lake to Gem lake, stay high and you will be able to follow a path. There a few divergences down to Gem Lake but all end up at roughly the same point crossing it's southern edge. The trail from Snow lake to Alpental is runable but a slow run since it's rather technical. I prefer the CW direction since you have an easy road to run the last 3 miles from Alpental TH back to Franklin Falls TH. If you do a complete loop from Franklin Falls you'll bag 2 FKTs. You can likely beat my times, since I was slowed by snow and insane crowds on the trail. I moved efficiently but it was mostly a casual jog/hike.
Mark Griffith's August attempt is a good .gpx to dl and use to route. My second attempt is close but still travels off trail due to snow.