I try to get a few miles out ahead of the convoy where the lead escort and first column are barely visible through the scope and steer directly toward them.
as they approach i can plot the position of the outer most rows on the map screen after a few sightings I can plot their course on the map screen using the ruler tool... I try to plot the course of the outermost rows of merchants. once i have a good idea about where the outermost rows will be I can adjust my course left or right as needed to end up in the sweet spot about 500-1000 meters off their beam.
I use GWX, not sure about NYGM but in GWX the flank escorts will break off on a search pattern every few minutes, this creates a good opportunity to maneuver into the convoy's 10 and 2 o'clock positions about 45 degrees off either side. just be sure to give the lead escort a wide berth.
another option - works the same way, but with a little more patience you can find yourself inside the lanes instead of just outside them, you just have to wait for that lead escort to pass by.
the approach can also be made at night on the surface in early war years. the pro to a surfaced approach is - enemy hydrophones cannot hear other surfaced vessels so they HAVE to spot you visually which is difficult especially if you control your temptation to rush in at high speed. most attacks in 1939-lat 1941 were made from the surface
the con to surface approach... you are not invisible to the naked eye and can be spotted and will then have to pull the plug and crash dive thereby spoiling any chance of attack but the destroyers will have a rough time seeing your little boat. additionally, in late war the escorts have radar so you have about a 95% chance of being detected and the surface approach is impossible unless radar hasnt come out yet
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