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.
Your broker in charge should be assisting you with this. The NC real estate commission puts out a publication called "Residential Square Footage Guidelines' that you should always keep handy to reference when measuring a house. You can order it from:
I know my local board always has copies of it forsale, you could also check your local board and see if they have it. Pick up a copy, read thru it, and then talk with your Broker In Charge regarding any questions you may have.
I've had similar discussions with new agents and agents that have been around for decades, and it does seem there is alot of conflicting opinions on how to measure in situations like this, and I'd expect you to get conflicting info from here as well. You safest bet is to get the book (it's a very thin yellow book) and talk with your BIC. Good luck.