fleming left hand rule is for motors:
however since all these topics come under electromagnetism and these forces reproduce each other we can use this law there also. also you can sometime think and analyse that the law used for motors and the lorentz force are one and the same
so if you strech your left hand and open thumb. fore finger and middle finger perpendicular to each other then middle finger represents motion of a positive charge, fore finger represents direction of magnetic field and thumb represents the force on that charge due to magnetic field.
here since we deal with vector product so whatever be the angle between magnetic field and direction of motion of charge (i.e. forefinger and middle finger) the direction of force will remain same perpendicular to the plane containing the other two vectors (i.e. always will be the thumb)