it would be helpful to move the definition of virtual tags one level up also, next to the definition of custom tags.
F.w.i.:
That button is also already available under Tags(2) > Tag Handling
And that's a more logical position, since it affects all tag fields, not only custom or virtual.
That it's also under Tag Definition is probably to draw some more attention to the feature.
Which makes sense, since I think to remember that I have been completely unaware of the feature the first years of me using MusicBee ;-)
edit:
What would improve awareness of this feature, is if it was indicated somewhere with the tag itself what format (number/date/string/enumerated/etc.) it is set to use.
Something like this could be an option?: