If I read this correctly, you determined empirically that the ratio of volume of oil to volume of air needs to be adjusted for the weight carried by the shock.
For your weight, there was too much oil in the shock and you bottomed out because you compressed the air to a point where the pressure was so high it couldn't cushion you any more. Lowering the starting pressure extended the range of compressibility, but not enough to provide a cushion all the way to the bottom.
You sat on the bike and reduced the volume of oil. This gave you a greater limit of compressibility. Extrapolating your experience, a heavier person would remove more oil when sitting on the bike. The same is true if you and your wife sat on the bike and drove more oil out.
I wonder if there is a limit to the amount of oil that can be reomved. How does the bike ride now when you go 2-up?
Finally, I'm guessing the oil/air ration in the 12" shock was different from the 13" shock.
For your weight, there was too much oil in the shock and you bottomed out because you compressed the air to a point where the pressure was so high it couldn't cushion you any more. Lowering the starting pressure extended the range of compressibility, but not enough to provide a cushion all the way to the bottom.
You sat on the bike and reduced the volume of oil. This gave you a greater limit of compressibility. Extrapolating your experience, a heavier person would remove more oil when sitting on the bike. The same is true if you and your wife sat on the bike and drove more oil out.
I wonder if there is a limit to the amount of oil that can be reomved. How does the bike ride now when you go 2-up?
Finally, I'm guessing the oil/air ration in the 12" shock was different from the 13" shock.