Calculating a sum in Adobe Acrobat Pro

I'm creating a fillable pdf for a budget to go on my company's website. I've set up the form and everything, and I know how to get the program to calculate a sum for me. However when I get to the Calculate field in the properties dialogue box and go to select what rows I want the program to calculate (in this case it's addition) it won't let me select individual boxes. The program is only allowing me to deselect all or select all. Has anyone encountered this  issue?

(Photos attached exhibit the dialogue box I am referring to)