That is a computed field. Input all the information for lines 1, 2, 4, 6, 7 and 8 and it should populate.
A way to find in put is Check Return > Forms > Click all > type the form in the search box > click the form and find the field (line or box) you want. If it is shaded blue, click on that and you will end up at the input.
Answers are easy. Questions are hard!
