when i did my
Channel Splitter thing i needed this as well:
$calc($window($active).w - $iif($readini($mircdirmirc.ini,nicklist,$active),$calc($gettok($readini($mircdirmirc.ini,nicklist,$active),2,44) + 10),105))
Seemed to do the trick for me and most other people.Though i wouldn't put my life on it whether its a failsafe method or not. Especially since that 10 value is for the scrollbar and 105 seemed to be the default width for the nicklist on my PC.
This all reminds me I seriously need to update that thing lol.