if you push above your current limit, try which vtt is better. It will show more stability. If until 0.9875v it still scale push more, until you see negative scaling, usually more vtt more stable and you can push more mhz until negative scaling.
//edit
Forgot to add something, default vtt is half of vdram, sometimes people just set vdram from BIOS and push more vdram it causing unstability, maybe its not vdram limit just vttddr limit, and some memory need more vttddr then vdram.