It's best to first find the highest point of the floor in the room, then measure up to the top of your upper cabinetry and make a mark if you will be using a laser to install, or draw a level line where ever upper cabinets will be installed.
To locate the center line of the suspension rail, simply measure down 56mm (2 1/4") from the upper line you just marked and make a second mark if you will be using a laser to install, or draw a second level line where ever upper cabinets will be installed.
Tip: The Stabila brand of levels just happens to be 56mm wide/tall, so if you use that brand of level, or you find your favorite brand just happens to be 56mm wide/tall, then you can position to trace the top line where the top of your upper cabinets will be at the top of your level, and the bottom line at the bottom of your level.
Cut your suspension rail to length to fit where ever upper cabinets will be installed, making any joints fall on a stud.
Center the screw holes of the suspension rail on this second line and screw the rail at each stud (you can shim the rail if the wall has bows in it). Attaching the end of the rail with toggle bolts if there is no stud in that location.