Jamie,
You should set the property in your local site.xconf file (this is where all your site properties should be set), these local properties are then propagated into your Windchill config files using the command "xconfmanager -p" from a windchill shell.
You will then be able to find the resulting properties in the target file pointed to in the site.xconf property line from above, so in this case:
targetFile="codebase/wt.properties"
So you will find the result in the file wt.properties in the codebase sub-directory of your Windchill loadpoint.
Rgds
Gary