Parachute Release System

So I am using a quadcopter and flying it to a specific target, but before it does, it will be released out a rocket pretty high up, 4000 feet AGL, I was wondering if anyone could help set up the servo release for our parachute system. We have two servos that are going to be used to release our parachute. I am planning to have the servo autonomously release it at 450 feet and have the drone start an autonomous mission. I have seen the parachute release parameter on mission planner, but I am not sure this will work for us. It will be in a rocket, so we wouldn’t want the servo to release it inside the rocket once it hits 450feet on its way up. The parachute will have a decent rate of 20 feet per second. I want to use both the GPS and barometer on our quadcopter to work together to release the parachute, but how can I program this on mission planner. I want to avoid using other sensors. This will all be done on an airfield. The servos would be plugged into our Pixahawk 4 flight controller.