menubar height in Qt Simulator

HI,
I am trying to adjust my mainwindow and centralWidget's height to be screenGeometry height -(minus) menubar height.
However the menubar height is same across all kind of Symbian devices. (30)

I am accessing the menuBar()->height() in mainwindow.

Also, whether UI pointer is necessary to update the main layout.
I am updating the UI after a xml fetch. i tried with callback methods but the UI is not updating even though the call method is executing. But it is working with the UI pointer.
I am trying to avoid the usage of UI files, as i want to compile my project to a library.

Any help?