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read this post first... https://steemit.com/economics/@chiefjay/where-does-the-money-come-from-part-2-of-my-steem-economic-model
I am actually going to get into SBD in my next post about the economic system, but long story short. Steem backed dollars are backed by steem in the vault described above. The very same steem that gives vests the steem value you see in the UI.

When the price of steem goes down, the steem value of SBD goes up. The debt represented by the steem value of all SBD in circulation can only safely go up to a certain amount of the total amount of steem in the vault.

I think the econ system has a way of correcting that. It has something to do with the split between sbd and vests paid for blogging and curation, but im still working on figuring that out. But basically until the system can catch up and put more steem in the vault to compensate for the higher steem value of SBD, its going to penalize SBD conversions to keep the amount of debt within a certain percentage of the money in the vault.

There really is no vault with STEEM in it, as far as I can tell...

There is a reward fund, and while I'm not entirely sure how it works, I think a lot of what you're trying to figure out has to do with that.

Also, where are you getting your numbers for D to ownership ratio? You get that the debt part is sBD, and the debt to ownership ratio increases when the price of steem decreases, right?

Yes... I was doing it with the value of the SBD in circulation, denominated in STEEM vs the liquid STEEM supply.