Assuming XMR follows Pareto's ~80/20 distribution, we'd expect the fat part of the distribution curve at the front representing whales, with a long thin distribution at the end representing small holders.

As the GUI lowers the barrier to entry, I speculate the long thin part will gain more than the short fat bit, because the higher barriers aren't as much of an issue for the smart/rich whales (and I believe whales tend to get into these kinds of projects early, long before Official GUI type stuff).

If we define whale as a second-order (top 20% of the top 20%) Pareto distribution holder, we still need an estimate of how many users exist to calculate an exact qualifying number.

Or we could say a whale is defined by the ability to make an outsize splash in the markets, IE significantly move the price in either direction regardless of overall market sentiment.

How about 1%?  It does sound a little too "Occupy" for my taste, but it gives us a convenient minimum of 120,000 XMR and there can at maximum only be 100 of them.