I’m using a Shelly Plug S with ESPHome. Calibrating this device is time consuming. Does anybody have a well set up calibrate_linear config they are willing to share?
Up to impossible. As all devices build on that dirt cheap hlw8012 suffer from the same (hardware) limitations. The power metering chip actually isn’t able to measure voltage and current at the same time (which would be needed to calculate real power) but it does it alternating. Depending on the power factor and how fast the load (take a PSU as example) switches it’s not really useful to try measuring it with a hlow8212 based hardware. 
Only use case comes to my mind is a constant restive load like a heating element. In that case the readings should be acceptable if no big voltage changes happen. 