I have calculated the apogee of a 2.5kN avg. thrust sounding rocket once ignoring drag and then assuming maximum constant (*magnitude and direction*) drag after burnout. 
Here are the figures I used in my calculations:

Rocket:
- Carbon body and payload: 6 000 g 
- Propellant: 8 496 g 
- Motor (N2501) at burnout: 4 604 g 
- Average thrust:  2501.8 N 
- Burn time 6.09 s 
- Total drag coefficient: 0.54

Other constants:
- Nominal launcher elevation angle: 75°
- Frontal area 𝐴 ≈ 0.0103 m2
- Constant air density of 𝜌 ≈ 0.631 kg m−3

To my surprise, the results were dramatically different (~47km vs ~4km). **Where should I expect the actual apogee to be within this range and why?**