fixed tax amount

When creating an invoice, there is an input for the "Tax" information. Currently, it allows the user to enter a "Tax Rate" in the form of a percentage, entered as a decimal integer.
However, it would be nice to have a sliding toggle to allow the user to enter the tax either as a dollar figure, or percentage. Similar to how the "Discount" input has a toggle for Percentage or Amount (% -> $). This would allow the user to enter a fixed tax amount if that is what they need, or calculate the tax as a percentage of the invoice amount.