Hi ....
why not use Wattman from AMD as a start, it' s included in the driver and its ok for undervolting.
Simply get in there and set the speed and voltage you want and ya are kinda good to go. No need to touch PL on AMD cards when you want to undervolt ... Normally you can get the Polaris down between 70 and 80W (Chip only) ... just depends on what you want. Good setup to start with is 1150 MHz and 900mw for the core, rest is some tweaking

. Get GPU-Z and check there what is drawn by the GPU ... or HW Info ... When the core is set, start to undervolt the GDDR5 ... step by step

Cheers
So I tried lowering each voltage value by -50mv it didn't do anything on the power draw, tried -100mv for each value and when I applied the setting it kind of reset the GPU as the application crashed and the display flickered for a moment