Calculated Key Figure problem

Hi Everyone

My first time on here so I apologise in advance if I am asking an easy question or I am in wrong area.

I am creating a query but having problems with my formulas

In my query, column C=Forecast Volume and Column D =Actual Sales

I am trying to create a calculated key figure where Column E would be a figure representing the difference between C and D

so if C=100 and D=50 then the formula would be E= C minus D but if C=50 and D=100 then E would be D minus C

It looks simple but I cant seem to get it right.

I also need Column F to be Column  E represented as a percentage

Any help would be appreciated so thanks in advance.