I was looking at the host of Monte Carlo spreadsheets on gummy-stuff and discovered that (almost) all dealt with withdrawing money ... after retirement.
But what if you'd like to add money, annually, for umpteen years, the amount increasing with inflation and ...

>To see how much you'd have when you retire, right?
Exactly!
So here's a spreadsheet:

You pick some annual return and standard deviation, an inflation rate and how much you'd be investing each year.

>And that investment increases with inflation?
Yes... but you can set inflation to 0%.
Anyway, after typing in the parameters you click a button and a bunch of Monte Carlo iterations are performed for a set of investment amounts.
The average final portfolio is shown as well as the best and worst.

>The worst looks ... uh ...
It do, don't it?