re: I just installed one of those smart thermostats. Quick question for anyone who owns one

51% humidity is fine - a properly sized and operating HVAC is designed to maintain near 50% relative humidity at peak heat loads, afternoon/summer, if the outside relative humidity is high. Of course during fall and spring, rainy days, or evenings when the heat load on the house is less and the HVAC is not operating expect relative humidity to increase somewhat if humidity outside is high.

Many home humidity meters only have an accuracy of plus or minus 3%. Usually constant humidity exceeding 60% is where concern creeps in regards to mold and health issues and you you need to start doing some investigating. LINK

Also not to rain on your parade, but be aware that according HVAC technicians Nest thermostats have a reputation of being problematic, some leading to expensive HVAC repairs LINK so if by chance you start having HVAC issues down the road don’t completely discount the thermostat as a potential source of the issue.