I think 정하다 and 결정하다 are not the best choice of words as they mean "to decide". "Is that based on your own judgement?" can be said differently. - 자기의 판단이 그렇다는 건가요? (literal and rather formal) - 자기 생각이 그렇다는 뜻이에요? (informal, everyday speech) 그렇다는 (short for 그렇다고 하는) is like "saying that/so", which is similar to "say something based on ...". 자기 (self) indicates the person being mentioned, so it can mean he, she, or you. We never use 너 to another adult (except close friends) as it sounds very rude like treating them like a kid. 당신 also sounds bad in a face-to-face conversation (it's okay in writings, advertisements, song lyrics, and so on). There is no common word corresponding to "you" for adults.

A: 저는 행복한 사람이에요. B: 행복한 사람이요? 당신 기준에서 그렇게 정한건가요? I think it would be more natural. 네 기준에서 그렇게 정한거에요? This sentence is a little bit weird, because 네 is 반말, and rest of the words 아 존댓말. We usually do not mix 반말, 존댓말 together.