I am so happy there is a nurse on board

I'm in need of some nursing expertise!
I'm currently doing my clinical rotations and the nursing instructors have been showing us how to administer IV meds. It was a little overwhelming at first, but I'm starting to get the hang of it.
However, I can't seem to grasp how to set the infusion rate if it isn't in the order. For instance, let's say there is an order to infuse a certain med over 20 minutes. How do you know what to set the pump at? Just thinking about it confuses me, and all of the nurses on the floor are so great at figuring it out....I want to be great at it too....
I asked my instructor about it and she gave me this long equation to follow....but none of the nurses seem to use the long equation. By the time I figure it out, it's 10 minutes later...ahhhh!
Can you help?