I too stress about my lights. However, If its just a little on the periphery of the cigar, it will usually catch without additional flamage. Take a few casual, normal puffs and recheck. If it hangs too much, I touch it up very lightly. I try to stress less and enjoy more

As far as the burn line, Its fairly normal for there to be a black line just before the ash. I would say that the black line is generally around the thickness of a dime, or about 1/16th to 1/32th of an inch. Are you getting a line much thicker than that?