What is taught at XJET is you have to set up the VNAV snowflake by going to the VNAV Decent menu and making sure that the Mach/indicated speed is set up correctly and the 3.0 degree is indicating as well for the descent.

Example: on some arrivals you have to read the STAR where it might tell you to maintain mach until transition to 280 maintaining that until reaching 10000 feet where you slow to 250. etc. then go to the PERF menue select Vnav, scroll throught the pages, where it is titled Vnav Descent, and plug in the numbers.

should look something like this:
.80/280

10000
250/10000 VPA
3.0 degree

once that is set up the snow flake will guide you through all the at above and at below along with in between until reaching your hard alt. you need to make sure you take your ground speed/2 add 0. example: 200 GS/2 add o. 1000 fpm should keep you in line with the snow flake with a little adjustments.....