Question

A. I saw that Maimonides and Sefer HaChinukh explain that the commandment generally forbids being drawn “too much” after one’s desires.
The Chinukh points out that these matters have no clear boundary, and therefore one does not receive lashes for violating this prohibition. 
Would it not be more correct to say that there is no actual “prohibition” here, but only a guiding principle? That is, the Torah told a person by what value to conduct himself, but did not clearly specify and say, this is permitted to you and this is forbidden to you; rather, the matter is left to each and every person according to his own understanding of what counts, for him, as excessive desire.
B. If indeed this is only a guiding principle, why is it counted as a commandment? How is it different from “You shall be holy,” which according to Maimonides is not counted among the commandments?
C. Another question about this same commandment. I looked in books by those who enumerate the commandments and did not see anyone count this verse as the source for a prohibition of looking and improper thoughts. True, in Shaarei Teshuvah, Rabbenu Yonah holds that looking at nakedness is forbidden here, but not in a general sense. (Even more so, in his Commentary on the Mishnah to Sanhedrin chapter 7, Maimonides writes explicitly that it is permitted to look at an unmarried woman for pleasure. Still, the focus of my question is not the practical ruling, but specifically whether the prohibition of looking is learned from “Do not stray.”)
Thank you very much!!