Very useful calculation, and very helpful. Like you said, most people (including me) will at first make that simple, but wrong, calculation - and then be in for a huge surprise when it doesn't happen the way the think haha.
The expectation / average wage is similar in the UK to the German one, so those figures will work here too. It's everyone's hope that Steem will rise high though, so if you manage to invest a lot of SP, then tiny powerdowns should be able to add a little to the total and hopefully :)
Otherwise, it's best to just treat it as 'extra' or an investment for the long term.
Yeah, I’m also hoping that Steem will rise again but I’m a kind of person who is more likely to calculate with the worst case instead of current or ideal situations.
And a powerdown is of course an option if you say for example that you’re satisfied with 10,000 SP and you payout the spillover.