This is in the rule I suggested. With a conditional in the actions, you can test for the current state--whether the switch (on/off) or level attribute, or something else entirely--of the light. (You could get the level and assign it to a variable, but I think that's over-complicating things here.) I might give one of those (e.g., this one) a look before diving into RM for the first time with something else. The docs are also good reading.
There are some videos on Hubitat's YouTube channel on creating basic and slightly complex rules. There's no denying that UI is clunky (it is subject to constraints that more or less originated due to decisions SmartThings made when creating their mobile app interface and the SmartApp framework).
