I am fitting an offset quadrant shower cubicle. Both straight sides of the tray are level but the curved part on the top has a gradual slope on it towards the middle. The bottom aluminium rail of the cubicle referred to as the bottom orbit in the instructions doesnt fit flush on to the tray. Coming away from the corners the gap between the tray and the orbit gradually increases to a maximum of 7mm near the drain hole. Is this normal? Does the bottom orbit of the cubicle sit on some kind of cement or plumbers seal. No matter how much I have played around with the adjustments of the upright side fastening posts the gap is still there. Any help would be appreciated.