Result1 is the internal result ID. The default is result1, result2 and result3. It's what the extension uses to identify the result set. It can be left as the default settings or you can change it so the formulas are easier to manage.
Label is the lable that will show up on the front end.
Formula is where the magic happens. Here you will insert the formula you want to calculate. There are a few crevats but mostly it functions like a normal calculator.
There are a few quirks to keep in mind.
Format is how you want the answer to be displayed.
Decimal places is the number of decimal places you want the answer to display.
There are 3 sets of results. You do not have to use all 3. Simply leave unused fields empty and they won't display on the front end.