It's not worth pulling out the graphing calculator from highschool
Your arrow hits. He's 80ft away. Though, you could rule that he'd be behind some half cover, granting him +2AC if he's standing on a solid surface. That's the way I'd do it.
Generally, I use theatre of the mind, so, the target is either in some range category or it's not. Thus it doesn't really come up as an issue.
If I really, really wanted to know the linear distance between two notional points, I would just calculate it via trig. It's not tricky. I would ignore the fact that firing up and down elevation is not the same as firing straight.