[Taneli]

GURPS Magic: The Least of Spells does define the Hedge Magician -talent for those at 5 points per level, and also talks about Magery with Hedge Magic Only -limitation that is set at -50%, but that is more of a talking point, than a specification.

These can be found at pages 19 and 20 of the abovementioned book.

Personally, I wouldn't go to lower than -50% with that limitation value, if you have the Hedge Magician -talent available for the 5 points per level -value set in the book. If you don't have it, I might be persuaded to go for something like -60%, but then I might take away the bonus to sensing magic items (in this case I would apply the limitation solely to the 10 points per level -cost of the Magery, and not to the Magery 0 [5] -part of the advantage).