First of all, check out this post.
http://www.ironworksforum.com/ubb/cg...c;f=2;t=019431
It's about Kensai's... but the level up information is still pretty relevant to you.
Most people tend to level up at L13, simply to get the extra fighter attacks.
Powergamers look at the maximum experience they can get, and simply pick off the highest level they can attain and still get one level higher as a second class.
e.g. With ToB installed and 8 million exp cap, we can get L21 fighter/L22 mage = All abilities of L21 fighter and L22 mage. Mmmmmmm - thats a tasty burger.
The OTHER way of doing it is looking at the special abilities of your fighter class (was it swashbuckler?) and see when you get them... it's usually every 3 or 4 levels. We also get weapon proficiencies every 3 levels...
Then just see how well they divide into the level you want (24 divides pretty well between 4 and 3, but its a little too high).
Check the benefits available to you before dual classing.