This page contains pretty detailed information about your question.
If you use the following options:
\usepackage[top=tlength, bottom=blength, left=llength, right=rlength]{geometry}
It should automatically manage the fields for you, but for your reference, all page sizes are measured from one snap in each direction, so the even field is actually determined from \ paperwidth, \ hoffset, \ oddsidemargin and \ TextWidth.