Alright, I found it...
When calculating the required bitrate to reach the specified size AVStoDVD takes into account if speedup or slowdown is specified. When PAL speedup is selected, but the script is later modified so there will be no speedup, then the bitrate will be off by about 4%.
To get the correct bitrate you need to do the following:
1. In Explorer go to the AVStoDVD\DGPullown folder and temporarily rename DGPulldown.exe to something else.
2. Fire up AVDStoDVD, under Preferences->AviSynth uncheck PAL Speedup.
3. Load your source, under View/Edit Title Settings->AviSynth edit the AVS script by inserting my script after the resize command.
Don't forget to rename DGPulldown.exe back to its original name when you are finished with your encode.
Cheers
manolito
P.S.
If you rather stick with the speedup routine, but you would like to add audio pitch correction (so audio will play faster, but at its original pitch), you can do this by using the AviSynth TimeStretch command. Let me know if you are interested...