Yes, I do get that but think about it...when you set your throttle in supercruise the ship auto-calculates your acceleration, braking point and deceleration based on your throttle setting. Therefore, the computer can easily calculate the actual, correct flight time because it is controlling the parameters involved itself. So why not just make it a proper, accurate flight time read-out?
You misunderstand what's happening here.
Your ship is not auto anything. It's not controlling your speed.
Your drive is reacting to the local gravitational environment. The throttle is not actually a throttle. It doesn't work like you expect it to. This is one of the reasons people have a hard time understanding what's going on. It's not working like the throttle on a bike or a car.
The throttle works as if it's in reverse. As if it's a forced retardation device. The drive wants to accelerate, it wants to do that a lot. If it can it will accelerate at its maximum rate. It can only do that when it's not interacting with a gravity field. This is why it accelerates slowly close to planets and stars but extremely quickly indeed when out in "flat" space such as way off the plane of the ecliptic away from the bodies in the system.
So your drive wants to go, but the gravity of a body is stopping it. If you add another braking system then that will give you some control over the rate of acceleration. In some places you have more control, your "brake" has a bigger effect. In others (flat space) your brake has a much lower effect per unit of throttle travel.
When you are going fast your drive is more sensitive to the gravity of system bodies, it can sense the body from a lot further away, and so its acceleration drops. It's not throttling back. It's that it can't accelerate because the gravity of the body is preventing it from running flat out.
If you had a drive without a brake and in an area of completely flat space it would accelerate at its maximum rate until it reached a point where it was consuming energy at a rate beyond its design parameters and it would then fail.
It's like having a car where the accelerator is always flat to the floor. You control the car with the brakes only. When your turn off is coming up (planet) you have to apply the brake or you'll go whizzing past it and have to turn around and come back again for another go.
It's not a throttle. It's a brake. You don't have pre-control, you only have post-control.
You do not do your thing, and then the drive responds to your command. The drive does its thing, and then you have to react.