It does sound about right mate. I do it roughly which gets you near enough.
Your 12mbps less overheads, is probably 11ish.
11mbps divided by 8 to change to MB = 1.375MB
Multiply by 60 secs then 60 mins 1.375 x 60 x 60 = 4950MB
4950MB divided by 1000 = 4.95 GB
4.95 GB x 3 hours = 14.85GB
He used the factors of 8 and got it spot on. I'm too lazy for that.