I got up a little early this morning. Got a cup of coffee, watched a beautiful sunrise.
Then I converted a recipe with my "bare hands" ...
The calculations are four function math.
The equations require close attention to the "unit of measure" (GPs, ...) associated with each number.
_edit (5/8/17): How to Brew, 4th edition, pp 54-55 has a well written approach on how to do this conversion. _