Granville-type Freighters Depth Question

[Murr44]

When using magnetic fuses I always seem to have trouble setting the correct depth to deal with these ships. If I remember correctly the GWX 2.1 ship recognition manual says 6.2m. Yesterday in the Med I attacked one that was being escorted by 2 escorts (an L-class AA & a Black Swan). It was clear & dark with 2m waves and I crept up until I got within 1500m. I set my #2 tube (TI) to magnetic with a depth of slightly over 7m. It passed harmlessly under the target. I tried again with #3 tube (another TI) set for just under 7m. Same result. Two wasted eels! #4 tube held a TII which I set to impact & a depth of about 3 & 1/2 m. This one hit the mark & sank her. We submerged and left 2 very "angry" escorts looking fruitlessly for us.

My question is: "What is a good depth to set your torpedoes for when using magnetic fuses on Granville-type freighters?"

Thanks in advance...