Thanks for reaching out. I'm going to be honest here, based on the target GMAT score you indicated, the quant aspect is the not the biggest issue. You need an overall score of at least 670 to feel good about the way your application will be read in the process. I have posted this before, so you can probably find more detail on it, but you risk being "screened out" (usually informally) if your score is below a certain threshold - usually 660 or 670 is that cutoff. What I mean is that readers have so many files to go through and its so incredibly rare for a GMAT score below their 25th percentile to be an admit, that often an admissions officer will just sort of skim (at best) a file with a score in that range.
If you can score above 660 or 670, that puts you in a range where maybe they wish it was higher, but at least they can't ignore you. They have to see what you bring to the table. And THEN you can start to worry about the way your profile will be received. Are you "ready" for an MBA - both professionally and academically (thus, the quant profile)? Do you have the traits they are looking for? Do you understand the DNA of the program in question? Are you goals powerful and persuasive? Are you showcasing transferable skills that will make you a badass in consumer goods? All of this becomes relevant the minute your file is going to get a fair and thorough read.
No adcom is ever going to say "if your score is below X, don't bother because we're not reading it carefully," but the logistics of the process pretty much dictate that this sentiment is true. When you are on file #60 for that day and it's a 630, you are pretty much thinking "okay, this person has no shot, I can read it in 15 seconds and move on." That's why you always here that while a GMAT score can't really "get you in," it can definitely keep you out.
I'd focus on upping that GMAT score in a more holistic manner, not worrying so much about showing quant skills, but just getting the raw number that keeps you safe. If you are thinking 650, then 670 is surely within range.
Finally, taking a course or two in the quant arena is a nice way to balance out your profile and show that you have those chops as well. However, someone going from advertising to consumer goods isn't going to be valuating companies or putting together actuary tables, so the only threshold you have to hit is "can he do the work in the classroom and not fall behind." In other words, continue to focus on your overall profile and don't drill down so much on the quant issue, which may or may not even be mission critical given your path. (Disclaimer: at schools like MIT and Haas, you will have to hit a higher bar on showing academic aptitude in the quant arena - those are schools that have proven at times to be more paternalistic on this point than most.)
Hope this helps. Study hard, get that score, and you'll be ready to tackle the rest of this daunting process.