Are you climb milling or conventional? Tramming the spindle is
basically checking the perpendicularity of the spindle to the table.
Using an indicator with "extended" type holder (see attached images) to swing a 180deg arc across the x (side to side) and y (front to back) axis. Optimal would of course be zero readings all the way around.
Gotta give a shout out to
www.cnccookbook.com www.cartertools.com
for the pics.