The configurations is pretty much OOTB settings. We are invoking the modal dialog using 'Flow In Modal Dialog' action, and running a flow in that modal dialog. OOTB the modal dialog opens in container-content section, I want to move it up to show more details.
The screen Header will be genrally 40 px. If you want your modal dialog on top of the screen header then you willnot be able to take the actions which are there in the header like logoff or exapnding the sidebar .
So suggeting you to have the same design and use a standard instead of fixed modal dialog so that it will fir to the content of the modal dialog.