I can't find your dad's post.
There are no #vars for DROs. However, DROs can read/write any #var you choose. Obviously, you don't want to step on #vars that do something important, so keep that in mind. You accomplish this by using the DRO's Parameter property. Enter 500 for #500, etc... The DRO would then display whatever is in #500 and when you edit it, it would change the value of #500.
You would typically use a #var if you also want to make that information available to G code. If you don't need that, then using a register as the backing store for the DRO would be better.
Steve