I am looking for a fantasy book published sometime in the 90's (I remember it being on the shelf at the same time as Reign of the Brown Magician, which suggests about 1995).
Alas, I do not recall even a fragment of the book title or author's name.
It begins with the protagonist constructing a series of instruments of increasing accuracy, which serve to point to the (three?) magical poles or wellsprings, which are sources of magical power.
He succeeds but finds a small inaccuracy. He persists in re-constructing his instruments repeatedly, trying for a perfect instrument. In the course of this effort (described as based upon empirical theories of how this magic operates), he makes a breakthrough, concluding that the inaccuracy is due to a fourth, additional pole or wellspring of magic. Potentially, if he gained access to and control of this wellspring, he would become a major power in the world, like the (three?) rulers controlling the known wellsprings.
In the course of his search, he makes use of a science of astrology, which allows for extremely precise queries of a distant person, so long as they can be targeted by an exact reference to their time and place of birth. At one point, he is targeted by such a coercive query by one of the magical rulers, who seems to be related to him (his father?).
At one point in the story, he halts his journey at a site where an army is encamped, imminently subject to tremendous magical attack, similar to a storm of fire. He acts to defend them via plowing a furrow around the field where they are encamped, making a symbolic ward of great strength.
