Yes, I considered that approach. But the vagueness involved in the notion of a "piece of junk food" was such that I found I had considerable latitude in determining how many pieces of junk food I ate on a particular day. By contrast, deciding whether I ate a certain type of junk food at all is usually quite straightforward, and not subject to rationalization. As Jon Elster wrote:

Kant’s rule of smoking only one pipe after breakfast was not unambiguous enough to give him full protection, since as time passed he bought himself bigger and bigger pipes. When feasible, the rule ‘‘Never do it’’ may be the only one that can be stably upheld.

(Of course, you may not have a problem if you are sufficiently self-disciplined, as you seem to be.)