Hi gmac - welcome to DNF ... That's a question thats been brought up many times before.
There's really no formula.
Some things to remember is that overture does not show results with less than 25 searches, while wordtracker will.
Many names with 0 wordtracker and (obviously) 0 overture still get type in traffic.
And there's domains with 30 - 300 or more overture score that get ZERO traffic.
Dont try to determine a formula, there is none.
As you get more and more into it, you will be better able to judge the traffic of a domain before you get a hold of the DNS.
Other popular tools you will find useful:
www.alexa.com www.marketleap.com www.google.com http://web.archive.org
Then there's still many more tools you can use, some free, some you have to pay for, you just have to dig around till you find what you need.
Kick back and enjoy the forum.
ps why'd you register back then and lurk till now?