I have depth perception problems, too. While judging how far out I am from the end of the runway is still difficult for me, I did get a good piece of advice to make sure you are in line with the runway and not on a parallel course too far out. Here it is:

When you first step into the "flight box" put the tips of both feet on the flight line and DON'T MOVE THEM. As you fly, follow your plane by moving your head or turning at the waist, but don't move your feet. When it is time to land, square your shoulders back up. If your toes are on the flight line and you are standing normally, then your shoulders are parallel with the runway. Follow the plane by turning your neck, and when it appears over your shoulder you are lined up with the runway. Keep it over your shoulder as you bring it in and you will be in line with the runway every time.