I agree with FFBob above. We add 50' to the MDA in our airline. In a jet you don't fly level at MDA, you fly the approach as you would an ILS, and initiate the GA at 50' above MDA (from a 3 degree descent), therefore ensuring you don't go below MDA. The logic is that if you were to fly level, when you broke out of cloud you would be too high too make the approach (due to the high speed), so we fly a stabalised descent as close to a 3 degree slope as we can. Most airfields we go to have an ILS, so NDB's are usually only used if the ILS is on maintenance on a nice day, so it becomes largely a visual affair anyway. Have yet to do an NDB down to MDA.