Well, to be honest, applying a 30 day period for every year for every planet point is an equally valid. That method and the method in which you take into account all of these other factors (for what reason? I see no actually logic behind it) are both 0% valid since each takes into account the motion of the stars and planets relative to earth, but neither incorporates the idea the motion of the stars and planets has absolutlely no effect on our daily lives whatsoever. So given that neither has any validity, 1) the method you described in the post with the numbers hasn’t got any more validity than the newspaper method 2) more people consult the Newspaper for their stars than will ever consult a guy who uses the other method (so the newspapers need debunking and the other guy isn’t worth bothering with) and 3) neither the newspaper method nor the other method will ever accurately predict someones future, but the newspaper method at least benefits from the fact that they can turn a higher rate of profit for a greatly reduced workload.
Additionally, if Dawkins then took the criticism on board and agreed to show that the other method is no better than chance, another astrologer could chime in with “Well, was the reading done while dancing naked round a thorn bush at midnight and chanting gregorian plainchant into a bucket? No? Well, it’s hardly surprising that it didn’t work then.” and when he debunks that astrologer as well, someone else will not unreasonably state that he was using the wrong type of calendar, and it becomes a never-ending game.