In Dashboards, just select options, Chart Formulas.
For more detail on any formula, click into the box. It will reveal increasing amounts of detail. You can also review a list of available metrics and formulas in Settings (top bar)>Metrics library (left sidebar). This will reveal an alphabetical list of available metrics and formulas. Use the searchbar to find the one that you want.
See also How to edit a metric and change the way a metric is calculated (how to change a formula)