I fly as an F/O on 742 and am a great fan of checking acceleration at 100kts. Though its not a simple calculation as altitude, wind, T/O weight as well as actual performance margin need to all be taken into account. I have an excel program which spits out a value for each Take off which I find accurate to about 3 sec. I start timing as we go through 60% and interestingly the acceleration times vary from 15 - 30 seconds ( I add 2sec for a static T/O ). The greatest variable I find is the F/E technique applying T/O thrust, a problem which won't occur on the new a/c using auto throttle for take off. As I'm in the right seat its will not be my decision to stop however I can at least inform the Capt if the acceleration is way off, possibly he may "feel" it too. I see it as a last resort strategy as I believe if you did mess the figures up you will "feel" that the accelaration is slow and now have a number you can compare it too. And can then make a reasonable decision based on the factors of the day.