p. 279
यन्मन्यते तं प्रति हृष्टबुद्धिः स वै तृतीयो ब्रह्मचर्यस्य पादः ॥ ११ ॥
In plain words
Whatever he holds to be so — and your mind brightens toward him rather than bristling: that, truly, is the third quarter of brahmacarya.
Close to the Sanskrit
What he thinks — toward him, one whose mind is delighted: that indeed is the third quarter of brahmacarya.