My understanding has always been that it’s based on powers of 10.
1,000,000 vests = minnow
10,000,000 vests = dolphin
100,000,000 vests = orca
1,000,000,000 vests = whale
The stats from @steemitboard are based on these powers of 10 and I’ve seen them mentioned elsewhere too. Don’t know if it’s official from Steemit, but it does seem fairly widespread.
Which works out to roughly:
500 SP Minnow
5000 SP Dolphin
50000 SP Orca
500K SP Whale
Good to get a general idea.