Thanks Encap for that quick little writeup. In regards to my accuracy, I'm not overly concerned about accuracy. I had a friend who did this same experiment on a 3-mile stretch of water, and he got to within 500 miles of the Earth's true diameter. For that reason I believe that I can do this to a recognizable degree of accurcay (I'm hoping to be within 2,000 miles of the correct answer). Either way, it will still be a nice experiment to do and I'll definitely report back on how it went.
Good luck!
Please also post your findings to The Flat Earth Society