As BaldEagl said, the map is divided into sectors that are numbered sequentially across and down so they are easy to find. Players break it down even further with the addition of numeric keypad location in the sector. ie: 12,10,1 is sector 12 across, 10 down and the lower left corner of the sector. I don't think you can get more precise, or easier to find than that.
And as Rolex said field numbers are not always sequential. They can't be, it's impossible. Somewhere on the map there will always be fields that are out of order. You'll always have say field 110 next to field 99 or some other random number next to some other random number. But, map makers try to limit this as best as they can.
But, to some extent Baldeagl is correct with his statment. If you find say field 120, odds are good that field 121 is nearby. The biggest exception is at country borders. This is because map makers usually try to complete setting one country's bases at a time (at least I do). This causes a jump in field numbers across original borders. So you wind up with field numbers like this... Bishops 1 to 65...Knights 66 to 130...Rooks 131 to 195.