I don’t know if you can understand Mandarin but FETC have a calc just like you describe at fare.fetc.net.tw/ The easiest would be to click on the first icon 地圖. Select your starting point on the map and click the red button, choose end point on map and click yellow button. Then click 開始計算 at the bottom. It will work out the cost minus and discounts and daily allowance etc. HTH!