quick, intercept my projectile idea and give it a nice bounce back!

You could have the foretelling to be reliable, except in cases when mages are actively involved in changing it. You could limit the range of foretelling, ie. a few days or weeks in most cases. You could have the planets actively affect the world, making sure their predictions come true, perhaps affecting their relations to magic-users.

Also, what kind of story is it? The way in which astrology is used in the story may provide a clue as to its justification.

It could be that people communicate with the planets themselves, instead of simply reading their positions, and the planets/spirits simply tell the astrologer what the future holds (perhaps from different angles, based on the personalities of the planets).

Have it work like this. The bigger the astrological body, the more general the prediction, but the farther it out it can predict. The smaller the body, the more specific the prediction, and the time it can predict out to is shorter.

So something like the local star(sun), might be able to tell you general things out to say a few months or a year, but a passing asteroid or comet might bring a forecast of a specific event in the immediate future.

If your astrology also involves the religion of your mages, you can give the planet/spirit beings aspects that will make them more or less inclined to sway in favor of a request for intervention. Um--I'm not sure that makes sense, so example to follow.

Planet X, or Spirit X, holds the aspect of love, but Planet/Spirt Y holds the aspect of fidelity and loyalty. If your mage were to request an outcome, such as the love of a companion, he would request intervention from X, but if he were also married to another woman, his request, and Spirit X's intervention would be opposed by Y.

I hope that doesn't sound too dumb.

Okay, it's not that I have something major to contribute to solving the problem, but I just want to say that it's quite a nifty idea you've got there.