Question Taichi z390 + 9700k: Vtt-DDR must be custom matched to the VDIMM?

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I've struggled for years to overclock my memory on this rig. I had always thought the Vtt-DDR should be set to 0.5 VDIMM. However, my Taichi z390 overvolts the VDIMM significantly (by 0.025V) and apparently doesn't have sensors to report Vtt-DDR. I had thought I needed more VDIMM even though I was already above 1.6V for 3995 @ 14-16-16-32 2T tRFC=325. I was going to give up but since there were no sensors reporting Vtt-DDR I decided to test various values of Vtt-DDR. What I needed for a BIOS set VDIMM of 1.575 (which is measured by HWiNFO64 as 1.6V) wasn't anywhere near 0.5 VDIMM though. I had to set Vtt-DDR 0.76V after testing every value from 0.7V on up. This seems to have cleared up all my instabilities. I can hardly believe it. Is Vtt-DDR THAT important or sensitive to VDIMM? Maybe the Asrock z390 Taichi overvolts the Vtt-DDR too.