This is just a quick guide for how to flash you RX470/RX480 cards. I hope it helps.
Use GPU-Z to see what kind of memory your card has
1, open atiflash, MUST RUN AS ADMINISTRATOR
2, click on save to save the original bios, save it as RX470ogBIOS.ROM or RX480ogBIOS.ROM (for convenience)
3, open polaris bios editor
4, click open at the top left and open the OG bios you just saved
5, edit the timings according to your cards memory
6, click save, save it as RX470modBIOS.ROM or RX480modBIOS.ROM
7, open atiflash again, MUST RUN AS ADMINISTRATOR
8, at the top select the card you want to flash
9, click load image at the bottom left
10, search for the modded bios you just saved and open it
11, click program
12...wait a minute to finish, when its done it'll say to restart...if you're done click yes, if you have more cards to flash click no and repeat steps 8-12 until you're done.
Also, when selecting cards to flash, it will only show 3 cards, so if you want to flash more than 3 cards, only have 3 cards connected to the motherboard at a time, flash the cards, restart, make sure it worked, shutdown, switch cards and repeat. And then make sure you have the right driver installed.
This is what works for me. I hope it works for you too.