Self-Organizing Man

Okay folks.

I need your feedback.  When do you use en versus em dashes?

I inclined to:

use the en dash between numbers, i.e. 35-45, etc.; and

use the em dash between phrases meant to set apart the phrase between the dashes - where I really want to make my point - as important.

Am I correct?  Or do I just pick one and use it consistently throughout the text?