The formula for estimated traffic is:
Search volume * Click rate of the site's ranking on the keyword
My site is 1st on the keyword insurance which has a search volume of 10,000 (10,000 users type this keyword every month).
The CTR of the 1st position is 37%.
So the traffic estimate could be 10,000 * 37 / 100 = 3,700 visits per month.
It is possible to enter your own click rates on the tool to calculate a traffic estimate. You must then access the "project edition":
Scroll to step 3:
Then click on activate. You can allocate different click thru rates for branded and non-branded keywords (provided you have tagged the brand keywords as is).