torpedo level / slope / 2" pipe

Im going to reveal my stupidity here... Im trying to figure out how to use a torpedo level to find 1/8 of an inch slope on a 2" pipe. I have been running the pipe and using the level where the bubble is just slightly encroaching one of the lines indicating I am not level but sloping in the desired direction. I don't have a 4' level. If I need an 1/8" slope, is slightly encroaching a line about right?