Random thoughts
1. Runways are allowed 2% slope. 430 mm height delta graded at 2% would equal a sloping ramp to the hangar door with a lateral base of 21.5 meters. This should be a an acceptable and affordable paving job.
2. I'd look at the statis slope limitations, nose-up and nose-down. Take whichever is less. Convert the angle to percent slope and you'd be surprised at the steepness. Example (working without trig tables here...approximation only): a 10 degree slope over a lateral distance of 10 meters would be a rise of approximately 1.75 m, i.e., an approximate 17.5% grade.
3. Why not find an acceptable compromise within the range of 1 to 2 above, and simply tow the helicopter in an out?