Problems with MP3 distortion-process

I am getting unwanted MP3 distortion (fraunhofer 320kbps) after mastering in Wavelab 8. Is there a target headroom that people are using to optimize MP3 sound quality? Or could it be something else?

I record at 88.1/32 bit float in Cubase 6, then export to Wavelab. Am I better off first thing doing a SRC in Wavelab to a 44.1/32 bit float conversion, and then working on that file to compress/eq limit/dither? Or can I compress/eq/limit/SRC/Dither the original file imported into Wavelab from Cubase all in one process ?

Thank you to anybody who provides experience/insight. I appreciate it very much

This should not happen, if the True Peak button is activated and does not light up.
The True Peak LED is based on a true peak analysis and lights up when clipping based on analog reconstructed values is detected.

Thanks Uwe. I’ll activate the True Peak button.