Hi shk
Take the logical approach.
You already know that your grid is AN55.
You just need to find the centre of 17 if you want to be more precise.
You could just take the centre from AN55.
Using your compass draw out the required distance.
What you do not know is how close you are to reaching that perimeter until you draw it out.
The radio message will give you the patrol start time.
This can also be seen in the Captains Log if you miss the radio message or do not receive one.
Further help can be obtained in TDW's Option File Editor Viewer and enabling the Player Objectives Tab.
The yellow icon showing below the camera bar is controlled by the player objective icon that can be found by opening the extended top bar.
A green confirmation pops up to tell you that you have completed the time for the grid ( not shown) and shows a new patrol grid (showing).
When all 3 patrol grids are completed the yellow icon disappears automatically.
Peter