Firstly, we need to know our GPU clock. For example, my ASUS EAH6850 is running at 790MHz.
Secondly, convert 79000 to hex, which is 0x13498. While in the vbios, it's actually 98 34 01.
If I want to get a 850MHz clock, I gonna convert 85000 to hex, which is 0x14C08 and 08 4c 01 in vbios.
Last, use Clover's PatchVBiosBytes to find 98 34 01 and replace it with 08 4c 01. And all done!
Unfortunately, I'm not able to boot my current Windows 8 with Clover now.
But I'm guessing simply reinstall graphic driver or Windows could bypass the problem. Because I'm able to boot Windows Install DVD through Clover.
These are the results to prove it works :