The cutoff value is the setting for the sensitivity adjustment– there is not any additional default level of filtering beyond setting a threshold on the rate of change of the infrared signal that is detected. Some other things that you can play with in the code, perhaps, include the amount of time spent sampling (num_acd_readings, as you noted), and the constants in the filteredReading function — which set the number of samples acquired and averaged to determine the slope. Playing with those values might require you to adjust the cutoff values as well.
Depending on the exact nature of the stray triggers that you are seeing, averaging for a longer period of time before determining the slope might help, or it might not.
Another physical modification that you might consider would be to add a sleeve around the IR sensors and/or emitters, that helps to narrow their field of view. You could use a piece of black heat-shrink tubing, even not-shrunk– just like a short, black section of a drinking draw.