Insert shameless MoBo plug...
10th Patrol of U53 - Radio Contact Intercept
In this SH3 patrol report, I happen to explain (with lots of pictures) how I got from an initial radio contact to a sinking ship using MoBo and basic Fast-90 manual targeting.
Of course, you don't have to use MoBo... I just like to.
Most of the report relates to approach. The entry for time 17:38 shows specifically how to set the TDC manually on the attack map for the typical Fast-90 setup.