Yes, you can put in a percent into the text box and in that percent text box's After Update event put:
Me.YourOtherTextBox = Me.YourOtherTextBox - (Me.YourOtherTextBox * Me.YourPercentageTextBox)
or if you don't enter it like .1 but as 10 (as in 10%) then
Me.YourOtherTextBox = Me.YourOtherTextBox - (Me.YourOtherTextBox * (Me.YourPercentageTextBox/100))