Greetings,
here is a reference for the western Balkans at the proposed start date of this mod;
As you can see, both regional/province and political boundaries are shown which should be useful when creating map regions.
Also, the black squares are free royal cities with attached mention of the year their legal charter was officially declared by the king, so they should provide for region capitals.
Croatia is, of course, under personal union under Hungary at this time, so all "pink-ish" areas(Croatia, Slavonia and Bosnia) are in fact a part of the crown of St. Stephen.