jotting down formulas

Great question, and great idea. I recommend exactly what you propose to my students, and the only time I've ever heard of a problem with it was someone who filled up her entire noteboard with her essay notes, and requested a new board for the quant section, so keep in mind that you can only have one noteboard at any given time.

Outside of that, you should definitely be able to jot down reminders to yourself during the essays so that you can use them during the math section (and verbal, too, if you write small enough to not fill up the pad before you get there). It's a smart move; I credit my "reminders" checklist to myself with quite a few points of my overall score. I wrote down a few keywords to remind myself of mistakes that I tend to make, and by keeping those in front of me I was able to catch them before submitting my answers.