I applaud homebrewing a controller. I made my own a couple of years ago and have been tweaking at it ever since.
My problem with using a controller such as you've decided on is that the fan is either 100% on or 100% off. When the fan is fully on it will blow a huge amount of air into the cooker until it reaches the target temperature. The problem is that after the fan turns off the temperature will continue to climb. The temperature swings will be very wide. I found that turning the fan on by varying degrees worked best.
There's a huge discussion going on over on the Virtual Weber Bullet board about this very thing. Some of those guys have gone WAY overboard with all sorts of software stuff that this hardware engineer doesn't understand.