Anyone who is shoved off a ledge should travel horizontally as normal and then drop straight down. So, if you are 5 feet from the edge and pushed 8 feet, you travel 5 feet to the edge, 3 feet further, and then straight down.
I'd also be fine with them moving an additional 5 feet while falling, but no more than that. A shoved enemy isn't being shot out of a cannon; shoves essentially cause a character to stumble backward - a sensible physical explanation is to prevent themselves from falling prone. So there really shouldn't be that much horizontal velocity that would translate into large distances when shoved off a cliff.
+0.5 for shove distances being affected by your Atheletics/Strength/the difference in the check between you and the enemy. I'm not 100% for it, but I'm certainly not against it. The latter in particular would add some fun uncertainty to your shoves - sometimes you'd shove the enemy 5 feet, other times 10 feet, and very rarely up to like 20 feet.
Difference between rolls wouldn't work because a person could roll a 1 with -1 Strength and no Athletics while the other rolls 20 with +5 Athletics, thus throwing them 25 feet, or something similar. On the other hand, a Strength 10 person could wind up shoving an ogre 15 feet just because the ogre rolled a 1.
But then, I suppose it might represent that the character caught the ogre off balance, and it stumbled backward 15 feet before catching it's balance. Hmmm...
However, someone with Strength 18 AND Athletics proficiency should probably shove someone a bit harder than a weak Strength 10 character. That's why I thought 5 feet minimum + Athletics skill modifier in feet. Then, what, maybe 15 feet is roughly the max distance shoved if you have high Athletics. Which, if you have that high of Athletics, shouldn't you see a difference between your shove ability and someone with 0 Athletics? Your skill and strength mean you know certain techniques and maneuvers that allow you to really shove someone hard.
