If you go to C:\Program Files\Adobe\Adobe Captivate 3\Gallery\Captions (or wherever Captivate was installed) you will find there are 2 files for each caption type. A .bmp and a .fcm file
If you find your caption and open the .fcm file in notepad you will see you can change the margins for your caption.
However, once you've saved you'll have to go back into your Project and re-select the caption so it reflects the new settings.
(I think, from memory)
Close, but no cigar I'm afraid.
Basically once a caption type has been used it remains cached in the Captivate project. So in order to see the adjusted margins, you create a new project and use the Caption.
Or, save all the Caption images and associated FCM files as a different name. Then because you are using a different name for the captions the margins are reflected.
I thought that would prompt a response from you, Rick.
It was quicker than experimenting.