Editing the stats file in GKnot
Goo to the Nandub files tab in GKnot.
Press Open and load the stats file XviD has created during the first pass (remember you set the filename in the codec setup).
Don't worry about the span and sensitivity setting, they're not used for XviD.
Then click on the Stats file editor tab. By default all the 3 of the options shown below are activated. For XviD we only need F-size Correction so make sure you uncheck both Motion Correction and Luma Correction.
Then press Calculate to calculate a scaled version of the bitrate curve.
Then press Save and you're done. If you're a true hardcore user you can with the Manual correction option to change the bitrate curve according to your needs but normally you wouldn't do that.