Calculating *x ROI from marketing spend

How much of this is an Excel question and how much is a more generic business question.

As somewhat ignorant of financial math, the first thing I did was put "roi for marketing allocation" into my favorite internet search engine. According to this (http://www.marketingmo.com/campaigns...on-investment/ ), the ROI is a simple ratio, which should be very easy to enter into Excel, (gross profit-marketing investment)/marketing investment. As simple as that should be, I have to wonder if there is more to your question.

Is that what you are looking for, or is there more to this?