To the best of my knowledge, we don't know with 100% certainty. But, if a few people sent them a variety of times and rD responded with race equivalencies, then I'm confident I could figure it out. Figuring out mathematical trends and relationships is what I do.
But, for a guess:
Most race equivalency calculators use the formula:
T2 = T1 x (D2 / D1)^R
D1 = distance you already raced
T1 = time for the known distance
D2 = distance you want to predict the time
T2 = predicted time for the new distance
R = performance degradation coefficient
When Riegel came out with this calculator he set R = 1.06. Almost all online calculators that you find use this formula with R=1.06 (Runners World, Riegel, Hansons, RunningAHEAD, etc.). Someone asked me what McMillan used and based on my original research it was "proprietary based on millions of data points". Then, given Riegel's formula I set out to crack the relationship between HM and M to see if it was consistent across the board. Low and behold, it was:
View attachment 312281
Instead of Riegel's 1.06, McMillan uses 1.07. We know that McMillan and RunDisney track very closely (within a minute or so) because very few people find themselves in a position that was unexpected (
@MommaoffherRocker's 2018 WDW Weekend comes to mind where we were expecting 5:29 marathon equivalent and she was seeded as 5:31 (per memory), thus just barely outside POT window per rD).
Vickers and Ian Williams have a very good data set that agree that the median for runners making a race equivalency is more like 1.15 for a marathon.
A New Race Predictor developed by Vickers: My analysis of the paper
Ian Williams: An Updated Race Equivalency Calculator Attempt
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But, a 1.15 adjustment vs McMillan's 1.07 is too large of a difference for that to be the case. So, my guess is RunDisney probably uses a 1.08 R adjustment.
View attachment 312287
That puts it within a few minutes of McMillan, and it's a simple adjustment from the classic calculator. To read this above chart, a 2:00 HM (in purple) would be a 4:10:12 per Riegel, a 4:11:56 per McMillan, a 4:13:41 per rD (guess), and a 4:26:18 median per Vickers/Williams.
If people were to ask a systematic POT question to runDisney and receive race equivalents, then I'm confident I could figure out their system much like the above.
You are correct, this is the way it has been done in the past (no guarantee the same system is always used in future years). If your POT seeds you in Corral B for the Marathon, then you will be in Corral B for the HM as well. This is because rD uses the same corral placement for each event with precedent set by the marathon seeding. You will find that your race equivalency for your HM time would have individually seeded you in a faster corral if running the HM solo, but because it is the same corral placement for both you are placed by the marathon seeding. Same goes for Dopey corral placement.
Some may ask why the discrepancy. It is my belief it is because they have a certain number of slots available in each corral that they desire to fill. They seed the marathon first. So the Marathon, Goofy, and Dopey bibs are placed in a POT pool and divided up into the corral pods. Then, those same corral assignments for Goofy and Dopey take up the same spots in the HM corrals as they did the M corrals. Then, they back-fill the remaining spots in each corral of the HM with the HM POTs. So when you see a lower POT for corral C, then would have been race equivalent from a Goofy/Dopey bib that's why. It's my belief that it's simply connected to the corral assignment process when dealing with bibs that need (or at least they choose to do it that way) to be the same per HM/M.