Why should one have to, as you say "You should just be in-game, doing stuff, figuring it out yourself" with things like the "686c" figure? Sorry, but there's a limit to searching for answers while sitting in the 'pit.
As Codger said, that number is your speed. It increases as you speed up and decreases as you slow down so kinda obvious.
And yes there are 2 speed indicators. In Supercruise it is mainly C, light speed.
In normal cruising speed it is meters per second.
Thanks. Are you hitting your <J> key at 7 sec or your 75% full throttle setting at 7 sec or both?
EDIT: Sorry its your 75% throttle setting at 7 sec as you've said numerous times before.
Must give you very little time to be ready to hit your <J> key?
Don't over think things. Test this for yourself.
For now, forget the 7 second thing.
Just approach a station (or whatever) and slow
right down once you are about 15 seconds away. Then just speed up a bit (you will have to judge your speed) and take your time.
Eventually, at about 1Mm (1 million meters) you will see the blue message pop up to disengage supercruise. It's that easy. Just experiment and see what works best for you.
Also on your left hand side (don't have an image) there are 2 indicators.... one shows the distance to your target, the other is your speed.
Once both of these are in the blue area, that is when you press j to disengage supercruise.