What you do is: you use the 'Grid Tool', you hit E, you chose this tool in the Map tools, you drag it where it covers the area. You can change its size to exactly fit the aerea of a coded patrol zone. Then you mark the exact coordinates with a supermark.
When you leave session, save your journal with a name, so supermarks are saved. You reload the journal when you reload the savegame.
All this assumes that you are playing with WoS or with a decent modded SH5 game having the NewUI_7_5_0 loaded, this feature is not there in the stock game.
Good hunting,
