Well, maybe I'm the only person writing about it recently, if that makes me an authority, so be it!
From our group of 8, one rode 35mm tires. He lives in North Bend, WA and rides gravel a lot, in fact, those are the narrowest tires he has. A very experienced gravel ride can get by with 35's. Referring to the
ridewithgps map, the two worst areas are typically the "warm up" climb starting at around mile 9.5, where you hit gravel on farm roads, then after 4 mile "wall", roughly mile 24-28, which borders the reservoir. In May it was pretty deep, but in July it had packed down a bit.
Use your own judgement, if 35 is all you've got, ride 'em, be careful on fast descents, eat your vegetables, yada, yada...If you have a choice, choose wider tires. I rode 42mm Hetres in May, and Rat Trap Pass fatties in August. I pinch flatted on the Hetres, but the RTP's floated on top of all gravel. I weight 200lbs, a lighter rider might not have the same issues.
Plenty of good info at
Oregonbike Bikepacking as well. If you're planning on doing it, download the map area using the ridewithgps app on your phone or GPS system, bring a water filtration device for use at mile 33, and then figure out how you're getting back from Tillamook!