Update column based on the changed value of another column
To set value based on other columns, see KB https://phpgrid.uservoice.com/knowledgebase/articles/355781-set-field-required-based-on-another-field-value-us
Basically you need to create an event handler using add_eventz(). More on event handler: https://phpgrid.com/example/custom-event-handler/
For custom validation rule, use set_col_customrule() function. See https://phpgrid.com/example/custom-data-validation/