Formulas
Arithmetic
Card → Enable Formula

Parentheses (Nested Cards)
Deeper Cards compute before their parent(s). Therefore, Nested Cards act like parentheses. They have an operator unless they are the first formula item.

Rounding

Presets
Card → Formula Presets
To remove a preset, re-click the active preset button.

Selectors
Formulas use the selected Entry Value.
Checkboxes
Internally, a Checkbox value is 1 or 0. This example multiplies that value by $5.00 (a "Hidden in Previewer" Entry) using a formula. Effectively, this is a no-code conditional form.
Alternatives to Formulas
The next document is about integrating JavaScript. Therefore, you can also write formulas like:

function ({ radius }) { return Math.PI * radius.value ** 2 }