In Descend via STAR, just use VNAV for happy & easy.
If not, usually controllers will give you descend instruction from based on 30NM for each 10,000ft rule unless he/she forgot you.
I prefer using V/S for 3 degree descent, the calculation is divide GS by 2 then add "0" to the end.
Keep watching GS, then recalculate V/S when GS has changed a lot.