Most people will tell you to shoot for 65-70% in the days of lockdown leading up to pipping.
Once the egg is pipped, a decrease in humidity will quickly (think heatshrinking plastic kind of quickly) shrinkwrap your chicks, thickening and toughening the membrane so they can't get through.
That said - I have one of those steaming machines used for cleaning. If I have a chick that gets in a tight spot and needs help, I turn on the steamer and get it warmed up. Once it's ready to shoot a steady stream of steam (try saying that five times fast), I'll quickly open the incubator, with the steam flowing toward the pipped eggs but far enough away to not overheat them, and quickly do what I need to do. I often find that, after that, my humidity actually _increases_ by a few percentage points, rather than dropping.
Also, once your hatching begins, the moisture inside the eggs will add to the relative humidity of the hatcher - then your humidity will easily go from the 65% you've been holding to 80%, sometimes, without you trying at all.
If you have any troubles increasing your humidity, remember that wet surface area is the key. Hot wet paper towels and sponges have more surface area than a simple pool of water the same size does. Also, if you can put your moisture source directly in the airflow, immediately after you heat the air, you'll raise your humidity faster. The hot air hitting the wet will pick up the moisture.