Creating conditional calculations

Just to try and explain a little more. I would need something that I would paste over the top of a cell to say substitute PRODUCTWIDTH with cell X2 and substitute PRODUCTLENGTH with cell Y2. Ive adjusted and re attached the spreadsheet to show what I mean, all the relevant areas say either PRODUCTWIDTH or PRODUCTLENGTH and I want be be able to easily change these words to whatever it says in X2 or Y2