You can either use a custom JavaScirpt calculation or the simplified field notation calculation, if your field names follow the required convention, with the “-” operator you will be able to perform subtraction.
However, we value your suggestion/ideas and we will have the engineering notified. Should we need more information inregard to this, we shall connect with you
Thank you for your patience.
An error occurred while saving the commentGary LoDuca commented
Here you go. It's simple. A subtraction function doesn't exist in drop down box.