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.