In the video on Rotax-owner.com it says something like approx, or about 3 liters. So next time try 2.7 liters. But for now it will probably work out fine after your first flight. It will read at the very top of the flat mark. If you want pull the plane out and motor the engine with the ignition off for 20 seconds. Then start it and run till temps come up to normal then shut down and check for leaks and the level again.
PS: What Scott said!