My general rule is that, with a loaded bike, 8% is about the steepest I will try to ride. Any steeper than that, even if you're geared to go slower, you'll have trouble balancing the bike. YMMV, riders and bikes vary in low speed abilities.
My low gears were in the low 20s, as are ours. There are places in the cascades where I resorted to walking. I was eastbound. Around the Diablo reservoir there were a couple of climbs that were a mile or so long, the first couple of miles east out of Tonasket, and a couple of miles on Sherman Pass.
I'd say you're geared low enough.