Since you mention the reflex sight I assume you're talking about the Recurve Bow rather than the Hunter Bow you get for free near the start. The following all applies to the Recurve bow.
1. Get a better sight
I found the reflex sight to be a modest upgrade over the default sight in terms of accuracy. Upgrade to the marksman sight as soon as you can. It has a distance range scale printed directly on the sight, with ranges for 0-50 meters, 80 meters, and 100 meters.
2. Practice > Math
But even though the marksman sight lists distance values, as IvoCoumans said in the comments, practice is much more important than math in this case.
Relying on numbers means using a conscious mental process: "Hm... that's probably about 40 meters away, so I guess I'll line it up... oh... let's say right about here near the bottom of the 0-50 line."
3. Develop line intuition
I use the bow whenever possible, and I find the lines on the marksman sight incredibly useful. And at the very beginning I thought about it more: "If it's pretty close, line up in the 0-50 range, otherwise aim higher."
But with practice, very soon you won't actually look at numbers and guess distances. It'll just be that something close-feeling goes on the top line, and something further-feeling goes on the next line, and so on. So it's more about developing an intuition of which line feels right for a given distance.