Not an answer to the specific question but a general observation - I kind of suspect that many folks may tend to over-humidify (I don't think 60% RH falls into this category) and we probably kind of foster that here with our emphasis on using humidifiers, etc. I kind of shudder when I hear people say things like, "I don't have gage but I just keep 4 humidifiers in the case to make sure."
Also, if you happen to have one of those cases with the built in hygrometer - ignore it. Those cheap analog gages are pretty useless. Just for giggles I kept my digital in my UkeCrazy case for a few days - I couldn't add enough humidifiers to keep the built-in gage at 50% - meanwhile the digital was reading well over 70%!
I agree with Chuck that I'd rather have an instrument a bit too humid than too dry - but I have seen a couple of ukes here that were obvious cases of "way too wet." In a recent example the owner had filed the bridge saddle down to the nubs trying to compensate for a top that was bowed up from too much humidity. He mistook the problem for a badly set neck, when it's far more likely the top had simply swelled.
Many years ago I had a classical guitar that I bought off eBay. It came from Florida, and had obviously been someone's beach guitar - there was even sand in the case and you could smell the sea. It was a cheap ca 1970 classical guitar out of Brazil but it had great action and good sound. By the time I had owned the guitar for about six months, keeping it in my office that runs about 40-50% RH, the top had dried out enough that it sunk to the point that the strings were almost touching the frets because the previous owner had filed the bridge saddle way down to compensate for a badly swelled top.
I think this is going to become more and more of an issue with the current popularity of cedar and spruce tops on ukuleles. Cedar and spruce have a much more open cellular structure than do traditional uke hardwoods like koa and mahogany and as a consequence they can absorb a lot more water and swell far more than the hardwoods. Combine that with the popularity of tenors vs. former high popularity of soprano bodies and I think we're going to see more and more ukes where tops have swelled enough to adversely affect action.
I have a cedar tenor that I bought used and I suspect that the top has swelled just a little on it from being over-humidified, though nowhere near as bad as the other cases I've mentioned. I'm keeping an eye on it, measuring the action every several days until it stabilizes, before I take any action to adjust the action.
I guess what I'm getting at is that you can't just "humidify on autopilot." You need to really watch what is happening with your ukes, especially those with softwood tops and larger bodies, and adjust your humidification as needed. When in doubt, err on the side of a little extra humidity. But, pay close attention to what's happening with the action and if your action is getting higher it's a pretty good sign you may be overdoing it with the humidity.
John