I’m not a vibration expert but I’ve seen a range of values for N from 4 to 5.3.
MIL-STD-810E, page 514.4-46 states that 4 is often used. In my technology area N = 5.3 is used.
When both acceptance (MFE) and qualification (MFE +3 or +6 dB) vibration is conducted. I compare the two levels and determine a reliability using the stress-strength method.