Hi

With a proper insulator on the back side of the heated bed (I use Nomex), and a good supply (I use MeanWell), and a good SSR (with < 0.25V drop) .... I get temperatures that are uniform to < 5C measured with an IR thermometer. This is with the stock heated bed (which is set up to compensate for the drops in the feeds). It also is true with a 12V or with a 15V supply.

If you try to measure the temperature of a moving bed, you need to be careful about cooling the bed with the moving air.....

Bob