Hi ual
All patrol grid objectives must be done in 1 gaming session without any manual game saves.
If you put an X mark in the centre of the your grid use the compass and pull it out to 250km; just make a note of the time when you pass through it and it will give you a good idea of the time spent patrolling.
You should get a radio message with a time and date start and also you have your Captain's Log.
Another option if you want to know what is going on is to set up the player objectives tab in TDW's Options File Editor Viewer. By using this you will always know when you have completed the grid and will show you the next patrol area if there is one.
In this picture by clicking on the yellow button bit I can confirm where I need to be.
250km patrol area marked out.
Here you can see the completed and next objective
Peter