In case you haven't seen UMMD's bed design, you can read all about it here. The bed is a piece of 300 x 300 x 8mm, MIC6 cast aluminum tooling plate with a 0.7 mm layer of PEI on top and a 750W line powered heater on the bottom.
In the images below, I set the bed temperature in the controller and left it for a few minutes to stabilize. The controller uses PID temperature regulation and drives an SSR that switches power through the bed heater.
In the first image, the controller was set to 70C, typical for printing PLA. You can see there is some offset between the controller reading and the FLIR temperature reading. But more important than absolute temperature, you can see that there is only about 3C variation in temperature across the bed surface, with some droop at the corners and edges, as expected. There appears to be a hot spot at the front edge of the bed- that's actually just a reflection of the hot-end.
In the next image, the controller was set to 105C, a temperature typical for printing ABS. Again, the bed temperature is a few degrees lower than the controller thinks. This time there's about 5C variation in temperature across the bed surface, expected because the higher temperature will cause more convection that cools the edges and corners of the bed.
It's hard to beat cast aluminum plate for even heat distribution. Between the flatness, even heating, and PEI print surface, I have very few problems with prints releasing from the bed before they are finished.