Ain't 8 tiles for 1 vertical as far as I could observe, more like 1-1 trade. The marksdwarves can shoot up/down at a 45 degree angle, and 20 tiles far tops. So 1 vertical is alright, they just won't be able to shoot straight down. This can be easily tested in arena.
Vision is wonky though, and you might get marksdwarves who don't shoot at all despite being in position to do so, because it's actually their arse that is facing the fortification.
Always carve fortifications into existing walls (constructed included) instead of constructing fortifications directly, and always build a secure "bunker" roof over them. Additionally, make sure your marksdwarves have no walking access to the enemy you want them to shoot at - no matter how remote that access might be, they will rush for it, if it exists, including climbing the walls/bridges and jumping over moats; thus, the easiest and most reliable way to prevent this are forbidden doors in the right spots. This together will prevent general problems with marksdwarf behavior.
There still remains a chance that, if shot at, your marksdwarves will dodge into and inside the fortification, which will give them the freedom to go mad and jump down into melee. The only way to fight it is adding [CANNOT_JUMP] and [CANNOT_CLIMB] to dwarven raws (but this can also make playing a dwarven adventurer unbearable).
Also, do not give them kill commands - only use the Defend Burrow scheduled alarm to ensure they stand in the correct spots; designate and redesignate the burrow as needed. There is no easy way to make them shoot exact targets nor even change targets at all instead of pincushioning the same gob all together, but you can use some wacky floodgate/door/drawbridge with a lever designs to break and re-open their line of sight, which will generally make them switch targets.