Actually, that Quality-based portion of the bonus exp *IS* only able to hit 300% if you use zero HQ materials. That was indeed what I was saying.

What I think you were getting confused by was the inclusion of rested, food, and any other bonus exp into that singular message given when you are awarded exp. If you have 50% rested, and when you 100% something to only get 320% bonus, the HQ materials you used *reduced* the 300% you could have gotten to 270%, which gets added to the 50% rested, for the displayed 320%.

That base 300%, separated out from any other bonus exp, can only be obtained with zero HQ materials. Use of HQ materials adds Quality before your synth - which reduces that overall bonus attainable.