Hmm, being driven crazy? All in the day for a Realtor :) I don't have the book in front of me, but I'd say measure from the inside, and add 6 inches for outside walls (as most homes are made up of 2x4's which are 3.5 inches wide, and then add the drywall and exterior sheathing and finish gets close to 6 inches on average) and add 3 inches for common walls (this was how I was taught to measure townhouse) Figure out the total area of the townhouse, and then for the stairs, measure the area that the stairs take up, and subtract from one of the levels. Add up and you have your sqft for the TH.
You are not expected to be 100% accurate, but you need to be fairly accurate in your measurements. As a learning experience, you might think about measuring the TH, coming up with your figures, and then hiring an appraiser to measure for you, but tag along when they measure and learn from them. I did this when I was first starting out and it helped me learn to measure. Last time I did this it cost around $75 for the appraiser to measure and provide me a diagram.