Everyone's humidity tends to be a little different depending on where you live. You'll have to do several hatches and see what humidity works best where you live. But, this is how I do mine in NC: I keep my humidity levels between 40 - 50% (usually on the low side of those) from day 1 thru 18. Then, I bump the humidity to 60 - 70% for the last 3 days by filling all the wells.
I just went thru this yesterday. After a small revelation ( and remembering I was told earlier ) You can remove one of the plugs to lower the humidity. Once I did that, the humidity was right where I needed it. The temp didn't drop, just the humidity. It's been holding steady ever since.
I actually run my incubators with none of the plugs in, this makes it easier to adjust the humidity when hatch time comes.
Humidity does greatly depend on where you live, I usually run mine at about 35%, unless it's quail or ducks, then it's at least 50%. But that's what works for me, might not be the same for you.
Normally, I just let mine dry out on its own, unless the humidity is really high, but I haven't had to add water to either incubator for a month at least, and it's been about 35-40% the whole time. The only one that needs water is the hatcher.