The mode field in NewScreen (16 bits) is not sufficient to take an entire mode id (32 bits). You have to use OpenScreenTags() instead of OpenScreen() and add the tag SA_DisplayID with the correct mode id. You can also use OpenScreen() with the ExtNewScreen structure and NS_EXTENDED flag set and add the Tags to the ExtNewScreen.Extension field.
You should let the user configure the desired mode id either in tooltypes or parameters or some config file. To get the mode id you can use the program GetModeID which is in the Visage archive on Aminet.
There is also a very useful tag SA_LikeWorkbench which clones the Workbench screen's mode and dimensions. Read intuition/screens.h and intuition autodocs for more information.
Bye,
Thomas
