I'd love to hear why you believe 6% is a reasonably fair expectation of annualised stock market returns going forward for the next 10-20 years!
https://www.elmfunds.com/blog/video-the-most-important-numbe...
http://www.fool.com/investing/general/2016/04/22/how-have-st...
Real returns 1950-2009 are 7% [1], so my numbers are already rounded down a bit. You might further adjust for, say Shiller 10 year P/E numbers, but how complex do you want to make it?
[1] http://www.simplestockinvesting.com/SP500-historical-real-to...