Here's a good reference chart that might be helpful:
How to use ~san, ~kun and ~chan in Japanese
I agree with Sashimister that "chan (～ちゃん)" is generally associated with closeness as well as age.
You don't necessarily need to use any suffix. Just using "Miku" is not all that unusual especially if you are just sending email.
Maybe when you get closer she may volunteer a nickname like ミ～ちゃん that friends call her.