You are 'spot on'.
Mileage estimates are the product of the marketing department. I have yet to see a scientific explanation provided with any mileage claim, therefor it's not possible to know how the number was determined.
Antenna gain expressed in units of dBd or dBi are at least claims referenced to a defined standard. The reputable manufacturer provides enough detail in their engineering documentation to give the knowledgeable buyer confidence that the antenna will perform at a measurable and repeatable level.
Here's a series of examples of how to use the data in your TVFR and antenna specifications:
http://forum.tvfool.com/showthread.php?t=109