I did not check your math, but its not that simple. There are other factors to take into consideration when trying to figure out how much your posts will be worth.
One factor to consider is how many people are using the blockchain. When the price of STEEM goes up, accounts which have been dormant will begin to post again, and many accounts will start posting multiple times. That means that even though there will be more STU's and rewards, there will be more people to share it with.