This is also going to change depending on gear level. At my current gear level, after a full duration of Arcane Power burn, I'm still over 50% mana.
If you look at this as a gaussian curve, then you can say that the midpoint value can be used to represent the average value. So if you burn to 30% mana during arcane power's 15 second duration, then look at the the midpoint which would be 65%, that gives you a value of 1105 mastery average value, multiplied by the 20% bonus of arcane power gives 1326. If you only go to 50% mana during the 15 seconds of AP, then you evaluate at 75%, which gives a value of 1530. If you are using this during your conserve phase with an average mana value of 95%, then you are comparing to a value of 1615 mastery.
Now, this is looking at only the values of this trinket and arcane power. Personally, I tend to use my mana gem at the same time as I use Arcane Power, and use this trinket. That extra spellpower then gets increased benefit from this trinket.
The TL: DR version is that if you are stacking more buffs than just arcane power and moonwell chalice, then depending on your gear level it is likely more dps to activate this trinket in time with the activation of the rest of these buffs.