I do it manual all the way, I only ask my WO for distance estimations. I feel this way I get much more fun from hitting a target. On a regular playing evening/night I am usually only able to attack 1-2 targets or perform only 1 convoy attack, cause my way of collecting data is time consuming, but that's ok with me, I don't need to engage multiple targets or eradicate whole the convoy in just 2 hours.
As it was said many times on SubSim forums - any way youplay a game is good, as long as it gives you what you want - fun (or anything else - frustration, stress). If you want to be invincible and sail with 50 knots speed and you play this way - that's ok too. That's just your style, no worse or better than mine.
As much as I encourage people to try manual targeting (check my signature), as I see it as more entertaining, as much I encourage them to just do what suits them best.