OHM, i'd go as follows - may or may not be the best, depending on your taste. I'd take Thompson/Cornell for the return trip, as the fast downhill on pavement seems a better idea-
- from NW Portland take st Helens Rd.
- L on Hwy 30
- L on Saltzman - it's a minor street so it's hard to spot, and also hard to X St Helen's Rd.
- climb - go pas the gate, continue on gravel (pretty good stuff)
- keep climbing as you pass Lief Erickson
- come out at Skyline (back on pavement), hang a Right
- LEFT on Springville
- follow to PCC
about 11 miles. Saltzman is a fairly gradual ascent & the gravel's reasonable. I've done it on 28c tires. I don't know how Springville is -- it's fine on weekends, but might depend on time/day.