Custom variance column on P6

Hi All,

I want variance column to show opposite from default ,example baseline start date is 1 April but actual date is 2 April .
The default variance , “ BL-start, “shows minus 1; I want to make a custom field to show the other way, +1 instead of minus 1.

I tried to work around in global change ,it works but the only issue I am facing is if any activity is not started or  throwing some random negative 5 digit number “ how to fix this ? “ Do you know the formula for default variance in P6 .

also i want remove the decimal units.