I think you misunderstand. 75% in supercruise doesn't mean 75% of possible speed- it puts you in the middle of the blue zone. In supercruise, this has a special meaning- the ship automatically manages your speed to arrive at the destination slow enough to drop SC safely. If you watch your actual speed (to the right of the scanner), you can see this happening.
The easiest way to deal with SC is to bind a button to 75%. Then head for your destination full throttle. When the countdown timer hits 7 seconds, hit the 75% bind. This will give you a very low effort and accurate approach- reasonably fast, but with no need for micromanagement. For bonus points, approach non-outpost stations (ones with a letterbox) from the planet side, where available, and you'll already be more or less lined up with the letterbox too.
I don't misunderstand why it does it but do not understand why the game doesn't explain why it happens. I went through the tutorials and it left out a lot of explanations. A proper manual would have helped because even the Wiki can be confusing to new pilots, even to experienced pilots of the original Elite... especially to them in fact.
I've been using the 75% button since day 2 of playing the game and the 25% one for going through the slot.
Oh and regarding the slot facing the planet... it used to be true in the original Elite but not true for all stations in ED.
I seem to remember the original Elite showed your orientation to the station in the left holo but ED doesn't. Another reason I hate those octagon cube stations... trying to find the entrance!