So, what holds you back on starting the DME update then?

A common situation would be that you see some nice DME coming into range. You may also see that your CDU could need some update.

Steps are
1) tuning that station
2) switching the CDU to DME update mode
3) entering the coordinates of the tuned station
4) checking, if the rough distance (shown on the CDU display) matches the DME measured one
5) starting the update

Takes some seconds, so even 500 kts GS (I sometimes go 1100 there  :P) can't hold you back there since the measurements (CDU value and DME display) are updated in realtime.
If you use that list feature I've described, you even get faster because you don't have to enter the coordinates as they are already pre-set.
I'd like to help you with that "aha" moment, Mark, but, currently, I can't see why it's not happening.  :-/