How to build this data validation rule, Planning 11.2.15?

Hello,

We do not have any forms that use data validation rules, so I do not have any example to refer to, and cannot figure this out. Maybe someone can help me.

We have a form that looks like this:

The layout is like this:

The business would like a validation on the Person_Days_Fed_Adj row like this:

Person Days Fed Adjustment <= (Mandays + ServiceDays)

Can this be build, and how?

Thank you, Kevin