I want to recover and update my 2X2 seater lounges by changing the existing straight unplumped 'squarish' dunlopillo cushions to the softer 'plumped up' look of dacron filled cushions. I think I should cut both the cushion casing and upholstery fabric a size or two larger than the existing fabric covers. Could someone advise me regarding whether this is correct and how much larger I should cut both the casing and the fabric to achieve both a good fit and the 'plumped up' look I seek, please.