Kass, I have one heck of a time keeping humidity up in my Brinsea at lockdown. I think 70% is too low though, I aim for the high 80's. The times that I've actualy been able to keep it that high I've had excellent hatches all on their own. In order to keep it that high though I have to fill both wells with paper towels and keep them soaked plus add a big cotton sponge and keep it soaked plus make sure the vent is almost fully closed. I drilled a tiny hole in the cover of mine and put a rubber hose (fish tank airline hose) into it. The end of the hose sits on the sponge and I have to squirt a LOT of water in there during lockdown to keep humidity in the high 80's. There's no such thing as too much humidity during lockdown if you've done a dry hatch. There's styrofoam incubators for cheap all the time at auctions, lots of people use those for hatchers.
Yes getting the humidity high at lock down is hard, I had both wells full and the vent almost totally closed, there was no room for a sponge, had it full of eggs.