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.
