Add the Percentage Price Field in a price set. (This feature will only work if you are using a price set on your contribution or event page.)
In your price set add a checkbox field and select the new option named "Field calculates Automatic Additional Percentage."
Selecting this option reveals additional options:
- Percentage: The actual percentage to be used in calculations, e.g, 3.5.
- Apply percentage to tax amounts: If checked (the default setting), this will cause percentage calculations to be performed on the total including any added sales tax. If un-checked, percentage calculations will be made on the total exclusive of sales tax. (Most sites don't use sales tax, in which case this setting will be inconsequential.)