go with google trends, identify a keyword which you rank no1 for in google, workout via a 60% / 40% split between PPC (depending on the keyword)/
SEO click ratio, that determines 85% CTR, the rest will goto competitors or other results. Then index it vs your new keyword and via the index you can work out the new keywords accurate traffic. That will give a very accurate result which completely differs from keyword tool or any other tool in the market.