I think you got all the pieces right. So, before that part, Shingo receives a message from Haru that says “近くまで来たんだ”. So, she’s saying she has come “close by”. まで, as you figured out, means “until” - she moved towards him (来る) until she was close by (近くまで).
Now, どの辺まで is referencing that. どの辺まで (until about where), means Shingo is wondering until how close to his office she has come. Or, in more natural English, he’s wondering how close to his office she has come (he’s thinking this because he’s searching for her, wondering how far he has to go to find her).
I hope my explanation is understandable. I find まで, 来る and 行く are not that simple to translate to English most of the time =._.=