Do you have the ACA maps? All distances on the route - including the Lowman Cutoff - are clearly shown in red type between red diamond markers to make estimation easy; just add em up!
Also, unless you'd prefer long pavement miles on a fairly busy highway I'd consider following the main route westward on road 579 through Bear Valley and heading south on road 555 (off-route) past Deadwood Reservoir rather than taking the Lowman Cutoff along Hwy 21. The climb of Scott Mountain is tough but you'll already have come over Dollarhide and Galena so no worries! You'd add some mileage and time but might still be able to make it work in seven days depending on the pace you intend to keep.
There are also more interesting routes (and lots of singletrack potential) heading back into Boise that would make your ride a proper loop rather returning via the Middle Fork Rd and Hwy 21. A handful of roads fork to the SSW off the main route between Placerville and Idaho City. Bombing back into town on singletrack and/or rowdy dirt roads beats the highway any day. The route of the recent Smoke 'n' Fire 400 might provide some inspiration. Here's a link:
http://ridewithgps.com/routes/5807789 Good luck with the planning and enjoy your ride!
