Excellent question. I too have the 899 HD SI unit. I place my x on the cover/structure they are relating too. The school moves, however what they are relating to will generally stay in place and will be easier to locate next time out. While the school maybe gone the next time, if you don't find the cover/structure then you are left wondering why you marked the spot?

I have several spots in the middle of nothing, one of these days I will delete them.

I made an observation over the winter while reviewing my waypoints, sonar recordings. Sometimes you mark a school that is traveling from one location to another and there is nothing around to hold them in place. This year I will try to find the two points of cover/structure that they are traveling between and mark both ends as well as the line between. Should help me understand migration patterns on that lake.