You have to set the Vcore to "Normal" in order to change the DVID.
From there, a safe spot for Dynamic is 2-3% of that of your Vcore...but that's just a very loose guideline.
It just depends on how high your OC is. If it's a very moderate OC, then maybe very little DVID is needed.
If the OC is very high, then you'll probably need to compensate for a lot of Vdroop with a high DVID increase.