The arrow height should be graded, (just like the mass building feature is graded, you cant even try to build 100 buildings in small area, only few).
And also it would be good to show the lines where the units will go.
Its really easy to make, just some math for the thing which says how large the arrow is.
This is sample image how it would look like:

Those white lines are the places where the units will be arranged.
Edit: explained more about 'grading'