Here's an idea that I have not personally tried, but it is similar to our field conditions. On each end of our runway and about 50-100 feet off the end we have a mowed patch of grass that is just the width of the runway and lined up with it. When we land, we cross over the mowed patch and you can tell where you are in relation to the landing area. Now I dont suggest mowing your field since I do not know how yours is set up, but you have indicated it is pretty open. So..go to Lowes or Home Depot and buy a roll or two of Surveyors Flagging. It is about 2" wide and looks like crepe paper rolled up, but it is plastic (Get the Neon Orange color). Unroll it on the ground starting in the center of the runway at the end and extend it out about 100'. It might need some weight to keep it from blowing around, but you can put nails through it and push them in. (Make sure you pick up afterwards.) Anyway, if you do this on the end you will be approaching from, you can tell if you are left or right of center "before" you reach the landing surface. BTW: My depth perception gave me fits for a long time. Just like everyone has said, give it time and practice, you will get there.