But I thought you said you aren't using LemEdit, so why does it matter what the default screenstart of LemEdit is?
Also, you're not necessarily going to put your screenstart at the center of your level anyway (often you put it where one of the entrances is located), so "nicely centering levels" and "setting the screenstart" are two different things.
There's a menu item in Lemmix that lets you turn on/off showing the screenstart position (drawn as a white rectangle). And you can zoom out, select everything by pressing A, and use arrow keys to move everything around. So both centering the level and setting screenstart should be a cinch in Lemmix.