Create formulas using duration fields e.g Original estimate * hourly rate.
Add or Subtract two duration numbers together from System duration fields, custom fields, or static numbers to create a new duration.
Multiply or divide a duration from System duration fields, custom fields, or static numbers; with a number or another duration field.
It is important to note that duration multiplier & divider formulas calculate results in terms of hours only.
Sum up a single duration field from your sub-task custom field and store the results in the parent issue.
Sum up a single duration field from your issue custom fields and store the result in an Epic.
An "Epic Link" field is required for the calculation to run on the child tickets.
If after adding a custom fields you wish to use to the project's Create and Edit Screens they should be available to be used in the formula as long as they are of a compatible custom field type. If the Custom fields do not appear as options when editing the formula then attempt to refresh the available custom fields by; navigating to the top right of the screen, clicking on the gear icon, and selecting the option "Refresh custom fields". Now the custom fields you've added should be available to use while editing your formulas.