How do I determine this node voltage?
Since voltage 770 V is defined as the potential between C and D nodes, you can rewrite it:

Vc - Vd = 770V

Vc = 770V + Vd

Vd will never be 0 because it is not directly tied to ground (or reference terminal). There is a resistor of 100 ohm between D and reference, and thus it will be a voltage drop Vd defined as:

Vd = I4 · 100 ohm