Here is a little tip for judging your archery distance. Get you some bricks and spray paint them with spray paint. Then put them in your yard at 10 yard increments from your target. I would recommend burying them in the ground with just the tops uncovered. That way you can cut grass without hitting them. Not only do they make good markers for shooting known distance and sight setting but you can go back to 50 yards and spend time visualizing what 10 yard increments look like on the ground.
It may sound a bit weird but when I have trouble with my distance I try to go to my memory and think about what it looks like standing at my 50 yard mark viewing my bricks back to my target. It works. The mind has the ability to use these types of visual cues, to memorize how far 10 yds is and then attempt to estimate how many of these increments there are to the target (i.e. the brick method above).
The second suggestion to find a spot that was halfway between you and the target. If you're not able to accurately estimate that distance, then find the point that is halfway to the halfway point and try to judge the distance. Once you find a point that you can accurately estimate the distance to then it's simple math to find the total distance.