Soda, the "+" represents the calculated lead according to the convergence you have set for the weapon. This does not mean shooting at the "+" won't give you hits if you are over convergence range, however, the dispersion is still a factor so shooting at a 600~700 yard range at the "+" will rarely give out any results if you are using a gun like the MG131. Thus, the lead sight helps accurate lead angle in acceptable ranges only. Getting the hang of "sniper shots" in 700~800 yard shots, or in vehicles like the 37mm Ostwind is just not gonna do with a weapon that drops a lot.
In the case of the MK108, a second "+" mark lights up. Thus, there are two lead sights, one for the MGs and one for the cannons.. and yes, it works.