My Mammoth trips were all scheduled: Jan. 1-3, Apr. 2-4, Apr. 17-18, May 25-27, June 19. My Mammoth skiing is historically biased to late season (thus only 6% lifetime powder by my definition), and even more so in recent years when I have been doing 3-4 weeks of mid-season destination trips. In general I do not believe Mammoth is a reliable place for powder skiing due to wind effect upon both the new snow and lift operations. 2009-10 was an exceptionally good season for powder at Mammoth due to lower water content (~10% vs. average 13%) and the middle tier of lifts (3, 5 and 22) being open during storms more often than in many seasons.
The definition of powder days is subjective, and was previously debated here:
viewtopic.php?f=10&t=6878
Since my primary method of counting is by vertical, I make my powder estimates that way. It is a stricter standard than most people's because I exclude the traverses and runouts. For example an untracked run in South Bowl is powder for about 3/4 of the Thunder chair's 1,000 vertical and a run out to Dragon's Tail would yield less than half of chair 9's vertical as powder. Even cat/heli places often have traverses and some common pickup zones that get tracked, thus I tend to count the better days there in the 90% range.
I know some people disagree with this, but I don't see a full day of lift service staying over 80% even on uncrowded and stormy days. Adam's powder definition tends to be even stricter than mine. He got a few of the big midweek Mammoth storm days in January. Those were in the 80% range because he got out early and hammered chair 22 nonstop for typically 25K or so and then quit around 1PM. His view was that even then it was getting somewhat tracked, so why not call it a day, then get out the next morning when he knew it would be deep and untracked again. I'm typically skiing the whole day, so a morning might be 70-80% powder, but after lunch you usually need to hit the Dragon's Tail/Bookends/Catherine's type places if you want untracked, so the day as a whole more often comes in at 50-60%.
By my strict definition about 15% of my 2009-10 vertical was powder. By previous discussions I've found where I count ~6K or more of powder, most people would consider that a powder day. There were 14 of those for me this season. In most seasons that percentage of days is typically about double the percentage of vertical and in line with the way most people look at it.