A suggestion - rather than making me choose an exchange up front, run the calculation on all the exchanges and show the results for each, sorted by most profitable first.
Very good idea. Thanks!
The tool now works as wormbog suggested: each exchange site can be enabled/disabled individually (all enabled by default) and CT Path Finder will show the top results (sorted from best to worst) among the enabled exchange sites. I think it is better like this; less unnecessary clicks.