If you are flashing back to CEX, then yes, use the CEX.bin renamed to "rflash.bin". 256mb is the correct size.
239mb or lower isn't a preloader NAND backup.
You can double-check the ID with a hex editor if you are not sure (ie., make sure its not 82)
You should have advance.cfg, lv2diag.self and rflash.bin ONLY on your USB.
Double check the advance.cfg to make sure it is set to write flash and not read flash.
The one in the archive I upped is already preset to write.
From here onwards, do NOT use the power switch to turn on/off. Use the front panel instead.