Hi Toni,
DDR stress tools is used to check your DDR3 performance, The real clock you run will be determined by your PCB , if your PCB design is better, accroding to previous experience,MMDC clock may be near to about 450MHz by using DDR stress tool , if the clock is above 450MHz, maybe data test will be failed.
Here 450MHz is just an experiential value, just an example, at the time, you can set MMDC clock to be 400MHz, it will be reliability for your product.
Hope above suggestions can help you !
Regards,
Weidong