Researchers tend to zoom in on the Renfrew Border and the Paisley-Lochwinnoch area as the source of the Scottish Caldwell family. They are probably quite right.
Don't forget the second most prolific area for Scottish Caldwell's is Ayrshire itself and there are references to Caldwell's there going back until at least the 1400's. Especially "of Todriggs" and "of Annanhill" both near Kilmarnock and others around Mauchline, Ayr and Dundonald.
Others are at Maybole, Kirkoswald and (famously for Ireland) Straiton (sometime known as "of Stratton").
I think this represents a branch in antiquity.
Even so the distance from Ayr to Paisley is not that great - even by the standards in ancient times.
I have also recently seen a reference to Caldwell in the mid 1400's at Ipswich in England.
For some reason the "English" family is not so well known or researched and I have no idea on the relative numbers of family members originating from each area.
Recently I found a family connection to Nova Scotia and did a little trawling and found that the name seems very prolific there. The more I research the greater the family. It seems more common outside Scotland than it is in its homeland.