gravity32,
have a look at this boeing 757 bulletin.
"
The amount of undershoot or overshoot depends on the amount of the barometric correction, when the altimeter is reset, and the elapsed time from altimeter reset to level-off. "
It may help you a bit in your quest.
Boeing 757 Boeing Bulletins :: FMC-16 : FMC VNAV Anomaly During Intermediate Level Off