Achieving a percentage discount is possible with the use of custom formulas. For example, if there is a discount to be given for the 'Billable Amount' , a custom formula can be made like this:

Customized Billable Amount = Billable Amount – (Billable Amount*discount%)

Looking at the above example in a numerical way where Billable Amount is $1000 and discount is 10%, it will look like this:

Customized Billable Amount = 1000 – (1000*.1)