When calculating the values in the additional section, first cancel out cards in your hand and foot with cards in books. When that is done, enter the number of remaining cards in each section. Type just the number or use a "+" if the remaining cards are in books. Type the number with a "-" in front if the remaining cards are in your hand and/or foot.
An instructional section is also included. Click on "Instructions" and swipe or tap through the instructional slides to learn how to play Hand and Foot.
This app is open source and it is available on my GitHub: https://github.com/BenAlderfer/hand-and-foot-scores