I like where your head it at. Doing the calculations and making sense of the inflation.

There is another variable you should think about. The SBD conversion is not taken into consideration. Over the last few months, the SBD conversions have created inflation over-and-beyond the normal STEEM creation rate. This now creates a new STEEM supply that inflation grows from.

Here is an example with simple numbers
Lets assume our imaginary model has 1,000 STEEM (quantity in existence) in which inflation is at 8.51% currently and decaying over time. Let's magically sprinkle 50 STEEM into the pool of STEEM supply created from SBD conversions. Now that new 50 STEEM will forever influence the future inflation because we are starting from 1,050 STEEM instead of 1,000 STEEM.

Here is the thing, even though over the last few months, there has been many millions of STEEM created due to SBD conversions, the amount of SBD conversions have seemingly stopped. As long as the price of STEEM and SBD climbs, people will HODL or Trade instead of "Converting". This may last for a while, but can conceivably happen again. If prices of STEEM and SBD remain high forever, then theoretically, your approach to calculate the billion STEEM supply will hold true because SBD conversions will not come back.

The real game changer is what you mentioned about growth and the future dwindling STEEM supply due to new users needing STEEM. That can easily trump any tiny amount of inflation. If we would double the amount of STEEM users/investors, then the liquid supply of STEEM can run very low driving price way up!