A lot of the time, I find that I'm overshooting my targets in supercruise. I don't want to approach too slowly, because I still want to get there within an hour, but I also don't want to overshoot and have to turn around and correct course. About where should I set my throttle to ensure I slow down in time to disengage at my target when in supercruise?
Here's what I, and many others, do: When the target is distant, set your speed to max. Keep an eye on the remaining time to reach your target. When it approaches 10 seconds, start to move your speed into the middle of the blue "sweet spot range" on the HUD. As it gets nearer or moves away from 7 seconds, speed up or slow down accordingly. It's not a big issue if you go to 6 seconds, you can usually slow down and recover. If you drop to 5 seconds or lower, you are probably going to overshoot, unless you are within the last few Mm distance.
Only a fool breaks the 7 second rule... (old mnemonic about the distance to leave between yourself and the car in front on motorways/freeways).
6 seconds to go is ideal. Lower than that, you're going to overshoot.
To get to a stationary target, I generally just hit 100% throttle until it clicks down to 8 or 7 seconds, then I hit 75%. Bind a key or something to 75%.
There can be times when you're going around a sun when it gets slower. Try to fly around the sun rather than close to it (ie if you want to go to a planet on the other side of the sun, don't aim for it. Going the longer way is quicker).
Note that distance is not important, only time to go.