The squares are very simple - it's a way of making very quick distance estimates
assume you have the thing set up to represent the situation
You take a range reading - let's say it's an easy number, 5000m.
you'll note that on the periscope layer there are 5 divisions between you and the target, so in our example each division represents 1000m
The shortest distance to target track is this distance here:
you can see this is 3,500m given that each square represents 1000m
If you doing a normal intercept course (holding the target to a 90 degree bearing) the following shows the approximate distance
which is about 5000m in this case
Remember, YOU assign the range value of each square to be convenient after you have your range reading.
Using this you can then calculate how far you need to move to be ready to strike the target