I've only based this number on efficiency of my computer and also just a few users who have visible computers on a couple of other projects. Thus as I wrote it's a rough estimation. If user X has 1060 card working 24/7 on project P, user X has RAC = 100 and project P RAC = 20 000, he generates 1/200th computing power of the project. Of course would be interesting to calculate these numbers more thoroughly.
Whether current network is equivalent to 2000 of those reference computers or 5 thousand, that is not really important. We get the rough idea of the scale of the network, cost, profit per pc etc.