How did you centralise the dialogue window?
You can edit pnl_dialog.mdl file to move the dialog panel but the position values depends on your screen resolution. IIRC the one I started with was the one from https://rpgcodex.net/forums/threads/neverwinter-nights-nwn-nwn2-modules-thread.34529/post-8440599
There are some issues with longer text (with my version at least) that I still haven't bothered to work out because those are rare enough.TBH, I haven't really tried to understand how the file works but just fiddled with the values until it looked OK.