A lot of experimenting. See the many posts on this forum about printing maps. All printers are different and browsers print differently, i.e. different margins etc.. Also, on-line maps (Google and all others) come in predefined scales, these are the "Zoom" levels. If you want to have maps at specific scales so you can use grid readers, find a scale (zoom), printer and browser you like for your team and mike/print your own grid readers on transparencies.