[doulos05]

Well, I can't help too much with the manual targeting questions, but I can help you with approach.

First of all, the name of the game in the approach is patience. My rule for it goes roughly like this:
Surfaced flank: No closer than 10 nm
Surfaced full: No closer than 7 nm
Submerge the moment you see smoke on the horizon (check this for yourself manually every 15 minutes or so)

At this point, you should be in a good enough position that your submerged maneuvers are adjusting for target track and escort maneuvers (and setting up your escape). You should be nudging the boat, not moving her. Try to keep her around ahead 1/3rd to ahead standard. Save submerged flank for dodging depth charges.

Now here, I leave my scope up all the time. I'll submerge below the layer and come to periscope depth periodically (to use the distance traveled to knots trick, make it a multiple of 3:00 or 3:15, depending on whether your game runs imperial or metric) to check their approach, update my fix on them, and make/confirm my target selections. When you pass the thermal layer, it's important that 1) you be close to bow-on to the closest escort (that is, your nose is pointed at them) and 2) you be at least 1,000 yards from said escort unless you're coming up for your attack. Depending on how skilled the escort is (something you can't know until he actually locates you), you could come to periscope depth closer, 1,000 yards is just being conservative.

Rising to periscope depth procedure:
1) Turn the boat so you're bow-on to the escort
2) Give orders for periscope depth
3) Man the periscope (You left it up, right?).
4) When you pass 100 feet, flood all tubes and open all torpedo bay doors (just in case the tactical situation has changed and you pop up with a juicy shot right in front of you)
5) just before your periscope breaks the surface, point your periscope at the nearest escort since he'll be the first to react if you're detected (assuming you made the correct turns, he should be at roughly 0 degrees.
6) When you surface, watch him for a second or two, then do a wide angle (that is, no zoom) sweep of the convoy. Look for big, juicy targets and pick one as your primary if you haven't do so already. Pop out to your map, make whatever marks you need to for your chosen method of manual targeting.
7) Update PK if relevant (actually I always do this so I know when I should start to surface for my final attack, I'll clear it before shooting if I'm using Dick O'Kane, but it's a useful reference when you're at 165 feet and all you have to go on are sonar lines)
8) Have one last look at the closest escort, has he found you? (spotlights, aggressive acceleration or conns, etc)
9) dive for the depths, man the periscope for the first 20 feet of diving with it trained on the escort, sometimes you can still make out his hull (especially if you have a clear waters environ mod)

Your surface to attack should begin just after the lead escort has passed. At this point, you're inside the convoy. Now, you need to start thinking about the guy on your port or starboard (the flanking escort). Hopefully you aren't directly in front of him (he usually more than 1,000 yards off the side of the convoy), but no matter where you are, begin your climb to periscope depth. Same procedure as before. Line up your targets, fire your fish (try to fire at your targets from farthest to closest if you can so they all arrive around the same time), and watch the clock. I usually stay at periscope depth because I like the booms, but the safe thing to do is dive for the layer again. As soon as the first torpedo impacts, ahead flank for the thermal layer (they know you're there, silent running doesn't help you nearly as much as the thermal layer will). Once you pass the thermal layer, rig for silent running and steer towards your exit. If there are 3 escorts, I prefer to exit out the rear of the convoy. If there are 4 in a diamond, I go out one of the sides of the diamond (between the rear escort and either the left or right flanking escort).