ok, so i understand the reasons behind evidence markers, but how do you know where to start? like in the episode turn of the screw, when they got done with cataloging the evidence, it seemd like all the evidence markers were all over the place, with no definable starting point. and another example, if you have spots of blood on the floor, but they're all close together, but not in a straight line, how do you know which drop of blood to place the first marker next to? see its the little stuff like that i have trouble understanding. do they explain why you have to do things a certain way in the textbooks?