A user on Reddit (/u/rj102) clued me in to a site that I had not seen before, http://peercoincalculator.info/.
The source code that generates the probabilities is remotely maintained, so it isn’t something that I can just look at the javascript and figure out how he’s calculating the values, but on the surface, it appears to take into consideration differences like size of stake and age (e.g. a 1000 PPC stake has a Y% chance to solve a block in X time, while a 10 PPC stake has a much smaller Y% during the same time period).
I’m going to email the contact at the bottom, James, to see if he’s willing to discuss the what goes into those probabilities. If anyone is familiar with them already, or if you, James, are reading this thread, please send me a PM. Thanks!