Northern blot + imageJ + homework help

[exothermique]

I'm not sure what to do so I'm going to explain what I did up to date and where I'm having trouble
So I have to analyze a northern blot using the computer program imageJ (the northern blots are either ADH in glucose/glycerol at 2 or 16 hours)
So I use the box thing to create boxes around my lanes (crtl+1, crtl+2) of ADH transcripts
then crtl+3 makes a graph appear in the example I got the graph looks a lot different. I have peaks and the example has valleys. but I'm told to find the area of the valleys so I kinda did my best in that manner.
So I'm left with data (that's automatically put into excel)
some of my graphs have 3 numbers attached to them, whereas some have 5.
(because of the number of valleys)
So I added all of the values of the area and put them into a table
Now I am given a unit of enzyme like a control that I'm supposed to normalize with. there's 4 lanes on this control different concentrations.
I'm not sure what to do to normalize.
I'm guessing doing the [highest concentration - 0 concentration (for area) -control ] / ADH found in lanes that are not control.
However, I'm left with a ruler on the image and really not knowing what to do with that.
So my questions are:
I'm I doing this wrong?
Why is there a ruler?