OK, this is it. My head is swimming. I looked back on the model from the dimensions, to the Cd in Rocksim, to my peak impulse calculation. I dug up another formula for the total impulse of a circular charge, fired in free air, and against a circular plate. I also got the material-specific parameters for PETN. I had started using another library reference service instead of Google books and was totally confused. I happened to look back at the Google version of the book and lo-and-behold the equation was not the same. Hmmm. The Google version actually made sense and after the light came on, I could see that the other version was consistent. It appears the latter used 'p' for pi in one place.
Given the earlier formula for the peak load, and now the total impulse, the duration and shape of the pulse is no longer that critical. So far so good. I get a peak impulse of 1,639,246 N and a total impulse of 811 N-sec. This works out to be a mid range J162200.
Now is where the final major guestimate comes into play. That is the percentage of PETN vs. binder. After the altitude number came out low, I arbitrarily upped my percentage to 70% (mid-range of what I thought to be typical). I now get a reasonable 168 ft. (close to half the alitude on close to half the charges). Not bad for backing into an answer :)
Clarification: This sim includes 3 PETN charges, but not the initial BP charge that was used to get the actual Hot Rod moving.
Project Orion Hot-Rod Revisited, Part 1
Project Orion Hot-Rod Revisited, Part 2
Project Orion Hot-Rod Revisited, Part 3
Project Orion Hot-Rod Revisited, Part 4