Dropdown Items linked to Percentage?

Hello,

I'm trying to create a fillable .pdf form for an expense report. The expenses appear in a dropdown menu, and in the column next to that, the user inputs the amount they spent. The third and fourth column are taxes (calculated based on the amount spent) and total (sum of column 2 & 3).

All of that works fine, but there is an expense that is only reimbursed 6.5%, so the total column would be the sum of the cost out to the user, the tax, and then 6.5% of that sum. This is only for one item in the dropdown, though. Is it possible to link this formula to that one item if it's chosen from the dropdown menu but keep the other items in the menu functioning as previously described?

Thank you.