You can certainly do this with RM, and even enter an offset amount if you like. Use the Hubitat Virtual Omni Sensor to set your baseline humidity. However, as you'll see in the comments from @halfrican.ak in one of these posts, that method doesn't account for seasonal changes in humidity, so it's better to use a real second hygrometer with an offset value to set the baseline humidity. In the second screenshot, I compare against my Ecobee thermostat with an offset. What I'm not certain of, is if the offset amount should be positive or negative, but @bravenel can confirm, as this is not a real use case for me personally.
However, there are two community apps for this purpose that you may find of interest too.
and
