With the constant bearing method, you don't have to use the 'scope to shoot. You can also shoot on the map only, using sound bearing lines and/or radar positions (if you are at radar depth). Just wait until the ship passes your constant bearing line on the nav map. Bear in mind that sound bearings originate from the screws, which are located at the stern of the ships...
A sea story to go with that: Recently I encountered a small task force consisting of three destroyers, running 12 knots. Heavy rain, visibility about 600 meters. I set up for constant bearing, for a range just over 1000 meters, and stayed at radar depth to detect them steaming to the firing spot - both with sound and radar. Got the first one amidships (I think, never actually saw him). It sank immediately, one torpedo is enough to crack up those small tin cans...
Too bad his colleagues were of the elite kind. Can't imagine what they had against me...
I fired at the second one when it crossed the constant bearing line, but it turned on me, so the torpedo couldn't hit. I immediately went deep, below a thermocline, and crept silently along at just over 1 knot to get away, changing course now and then to confuse them... No chance. They seemed to know exactly where I was, circling above me, and dropping DCs right on top of me. After hours, and after sustaining an increasing amount of damage, they backed off a little. I tried to go to periscope depth for a possible shot... BAD mistake, they where back right on top of me in no time, and bombarded me again, this time for critical damage. Good tactics on their part.
Getting desperate, I surfaced and tried to run for it, flank speed. But they are expert gunners, you know... I got one with the deck gun, but the other sank me. Ouch!
So, learn the lesson, and avoid task forces consisting of destroyers only... It might be their "task" to sink unsuspecting subs like you. They are just waiting for you to shoot at them - you will be lucky if you escape alive, certainly not undamaged. Great addition to the stock game, keeps the tension up...