Currently it is possible to use calculated statistic value in Equation and Formula Smart Rules.
In Variables section, select device and pick one of its predefined statistic values:

Equation: Turn on vent if humidity in bathroom is 10% higher than in other room
Variables:
Hu Humidity in bathroom - Average by hours
Hu1 Humidity in other room - Actual value
Equation:
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Action:
Vent → Turn on