This is like measuring how many licks it takes to get to the center of a lollipop. There are too many variables to get a reliable number and even if there was a reliable number it is unlikely that it would help your business.
You need to take into account all of the keywords including the keywords you are unaware about. How do you count all of the long tail keywords you don't even realize exist?
You also need to account for all the referring websites. Not all traffic comes from search engines. In some industries a large amount of traffic comes from referring websites.
Not to mention taking into account direct traffic, traffic from overseas and the seasonal/monthly traffic fluctuations. Current events and the weather can sway traffic levels.
The marketplace is an organic creature that does not stay still. You need to guess what the consumer vocabulary will be in the future. If you built a website 10 years ago dedicated to dial-up modems I doubt it is still doing well.
Let's say you can somehow buy enough isp data logs, and data from browser companies and spyware companies. What difference does it make if you have 37% of market share or 42% of market share?
IMHO - What is much more important is your profit margins and your long term plans to keep visitors happy. Many websites were bragging about high traffic levels and then Panda & Penguin came along and reminded them to be humble.