Massimo Vignelli imparted this wisdom to me a couple years ago while I was working on my NYC Subway Map project at Parsons. I’m not sure if it was imparted unto him at some point too, but it was his rule of thumb for designing the signage for the MTA. It is equally valid and relevant to the work I am doing now as a UI designer for mobile apps, and I imagine it could be just as helpful to you in whatever your trade may be.