CALIBRATE YOUR MACHINE
Get your machine tweaked and running perfectly in no time!
I found a video that goes through it nicely: (Oct2016)
LINK TO PRUSA CALCULATOR
GRBLCALC GRBL settings calculator for calculating steps/mm
FEEDS & SPEEDS
The first discussion is here
Since updated and simplified in the C-Beam discussion
(with both of those links you may need to reload the page to get it to go to the correct post since the threads are long).