Providing everything is correct on the car (right dampers, wishbones etc), then rely on the Spring manufacturers measurements. Don't get me wrong, you won't necessarily get 40mm out of a 40mm set of springs. Try and ignore where it sits now as it could have saggy front springs etc.

Further to that, I would go 40mm maximum if I were you. Anything over that and it will want lowered shocks (best to do that anyway) and it will be awful to drive.

Hope this helps.