You got the basic tactic down, it seems hard, but its not that bad with practice.
It will be very helpful to have someone using C3 to extend your radar targeting coverage.
I usually have the large map open the whole time looking for a target, then lining up the shot only after I have caught someone standing still in a location that won't block the LT shot.
IMHO There are 4 main categories of hitting targets with the LT, in increasing difficulty.
Type 1 - Hitting a Stationary Target on the radar.
Find a target on radar, look at the range to that target. Align your LT gun so the Orange circle hovers directly over the target, and the range indicator near your reticule is the same distance as your target. Check to ensure there are no obstructions via the blue circles and fire.
Type 2 - High Angle shot on stationary targets -
This is exactly the same same Type 1, except you aim your LT barrel way up, over 45 degrees, past where it lists the maximum range (~1500 meters) to where it starts counting down again. These shots take much longer to land on a target, but are much less likely to be impeded by cover. This is my preferred method of firing. Rather than line the target up with the orange circle, you rely on that little line on your radar that indicates what direction your turret is pointing to aim.
Type 3 - Hitting a moving target
Just like Type 1, except you predict where they are going to move by looking at the "time to impact" reading on your reticule and guess at the speed the target is moving. Be careful with this one, you might hit a friendly. Don't bother trying to hit a moving target with the high angle shot.
Type 4 - Hitting grid locations
Rather than using radar, you rely on the map grid and team-mates calling out locations. Eg. you know you are at Grid position B4, Your target is at the upper corner F4, and this particular map has 400 meters per square, so you know you need to shoot 1200 meters in the general direction of the upper corner of F4. This is really hard to do. But some people here can do it reliably. It helps to have teammates spotting for you so you can adjust your shots.