Sometimes I still do it that way, but if one is more complicated, I use MapThis. Basically it shows you the graphic, lets you draw boxes on the graphic. Then you can adjust them and you'll see exactly where the zones are. You then name them and the program figures out the code (and all the position values). It's not that useful for maps that are uniform, but it's great for ones that aren't.
Here is the link if anyone wants to check it out:
http://www.zoniez.com/Newbie/mapthis/mapthis.html