Is it possible to display a CEF’s discount to NAV in VV, through a custom field? If so, how do I do it?

I’ve noticed that all of the CEFs show their price and value in 1:1 parity, and the RV is always 1.00 for CEFs. So it’s hard to figure out which of them offer a good value.

If I can display a custom field for their discount to NAV, that would help.