The wrench science is a good starting point, but not the end all of getting the correct size for a bike. It uses common formulas to figure out the bike sizing, but this does not work for everyone. Go for a double if you don't use the granny
Honestly the weight savings is very little when going double, and then you dont' have that granny gear if you ever need it in the future, but does provide a bit smoother shifting. Basicaly a trade off you have to figure out what is more important to you, ability to use it in the future (which you might never need), or smoother shifting and a slight weight savings.