777AV8R,
Sounds like we do it the same way maths wise. By the way, my 5* Groundspeed is the same as your way. (i.e. 5 *180kt = 900fpm). I work it out the same way as you anyway!
The director never says 'this is a CDA approach' That would help. The track miles given with the descent clearance is the cue to fly a CDA, but they are not always possible due to traffic and procedure design. Am I right? You can end up levelling of somewhere on the DW vector (at STN), but you can do the CDA from the next descent point.