How to do a McKinsey-style source of change chart

Some numbers today. The source of change is a tool to explain the delta between two numbers in terms of its components. Assume you need to get the story below across in a crisp presentation.

The first thing is to understand what's going on. Get some more information until you have the full picture in a clear table.

Now let's do the analysis. This is the tricky part, the text below does not do a good job in explaining this, you can click the spreadsheet for a bigger and more visual explanation.

1. Calculate the profit in the "before" scenario using a formula that just uses inputs
2. Now stretch each of the variables that change to their "after" value, jot down the value, and return the value back to its original number
3. Repeat for all the variables and see what delta in profit you managed to explain.
4. Calculate what is left to explain, and allocate that to the individual values.

Finally put the values in a nice waterfall chart.