I have been religiously following/recording postmarket, premarket and "Opening" data for GOOG for the past 2 years and can say unequivocally for GOOG, ONLY, that there is no necessary or consistent relation between what goes on in the after hours markets (postM and preM) and what the opening price is. Zero, zip, nada.
I agree with a previous poster that the OP is a critical price but there are problems in actually finding out what it is - real problems. Some days the GAOOP (Generally Accepted Official Opening Price - my acronym) is = the NOOP, especially since Reg NMS moved into the "post - July 9, 2007" period. Some days, the 5 quote services [Bloomberg, NASDAQ-Infoquotes, Google Finance, Quote.com and Prophet Finance] which I check every morning and my 2 data feeds [eSignal and DTNIQFeed] have the exact same opening price and it is = the NOOP.
On the other hand, other mornings, like this AM, I find something like the following:
Bloomberg and Quote.com = 519.01
Google Finance and NASDAQ-Infoquotes = 519.10 = the NOOP
Prophet Finance = 519.13
So tell me, what is the GAOOP today?
I have noted in the past that when Bloomberg and Infoquotes disagree, I am usually unable to find the "Bloomberg Price" in the trade-filtered T&S data I get each AM from the "NOII" link at the NASDAQ site (by subscribing to the NOII service you get access to a "Stock Stats" page which has chart data for NASDAQ-listed stocks like GOOG and as well a "Time and Sales" page which gives you BB/BA trade-filtered T&S data). I've included a printout of this AM's data around the RTH open and as well some comments about today's data when compared to another day's data.
These are not isolated findings. I have seen this sort of thing many, many times. There are other important questions which arise from these data, IMO, but I will not get into those right now. Suffice it to say that trying to "predict" the opening price from the premarket is not a good idea and is an excellent way to lose money.
lj