@Stan_Silverman, just wanted to let you know the Variables related features have been released to SharpTools.io today, and it does include the features to allow you to set a variable value using a device’s attribute value, as well as using the variable as the argument when sending the command to a device. In your scenario, you will be able to set $LuxLevel variable’s value using the sensor’s illuminance value, and set the virtual device’s level using this $LuxLevel variable as the argument. (Or you can simply remove the need of the virtual device and use the variables across the SharpTools platform to keep things simple.) 
See below for the release post today, and let me know if you have any questions.