This guesstimate would be just that, a guess.
You cannot assume anything. What spot on the first page are we assuming we are on to guess as to the traffic?
If you are first on the first page you get one result, if you are fifth, you get another number (guess)...if you only make it to the third page, well we get a much different number.
All I wanted to point out was that your guess assumes a great deal - You should worry about that when you have arrived on the first page... Then you can come up with some 'real' numbers... These traffic guesses have to be based on something solid... like which position am I checking from? Am I assuming I will make the first page for my phrases? Well now you're just being naive... to assume you will make top rankings on all your phrases and take a guess at the traffic based on this - IMO - that is backwards.
But that's just me -
It also helps at this stage to see who is holding the top spots for these phrases and then maybe check Alexa and Compete or Quantcast and see how they are doing with traffic too. It will give you an idea... but that would only help if they are targeting the same KW phrases as you are.