Your calculations are reasonable. But you should take some factors into consideration:
1 - Steem is expanding daily and the number of users and posts is booming and expected to continue like that.
2 - After hf19, the rewards has increased dramatically.
Best of luck.
Both are true currently, but not guarantee's for the future.
Thanks for engaging me on these topics - they're important discussions and I really appreciate the thought you're putting forth towards this!