Admission to Cambridge (& Oxford) is a series of hurdles. Once you cross the hurdle how much you crossed it by doesn’t matter very much.
Hurdle 1: ACT & APs (and for Econ at Cambridge, specifically including AP Calc BC).
Hurdle 2: Personal Statement & LoR. These address how suited you are to the Econ course at Cambridge.
Hurdle 3: the Econ Admissions Assessment. You can’t take that until next Nov, and the score is used to make the cuts for interview.
Hurdle 4: Interview: like a cross between a supervision (tutorial) and an oral exam.
You have to clear the first 3 hurdles to get invited to the interview. The interview is make or break.
tl;dr: 34, 35, or 36 isn’t going to make a difference in whether you get invited to interview.
The success rate for Americans averages around 7%. Scores breakdowns aren’t released for US applicants. Fwiw, Economics has about a 14% success rate- the lowest, bar architecture, of the humanity subjects and about the same as engineering and comp sci.