As far a zeroing your file you cannot break the beam until the drivesaft is moving so thats the best place. As for "premature" readings on the G meter coule be the way it is mounted, where it is mounted, number of magnets on the driveshaft sensor etc.

Here is a file from the heads up car. For those who don't know this is a 2800lb naturally aspirated small block on a 29.5x10.5 M/T slick. As for the Op's questiom yes it happens but it is almost instantaneous. But it also depends on gearing, converter, trans gearing etc etc. We launch at a relatively low RPM to give the converter a "run" at the car. In our case we are talking 1/2 second before we have movement from the DS to flash of the converter. In the case of this screen shot on that day in St Louis with the conditions was 9300, a tad lower than I would like it to be and it got short shifted so in theory it was really tight for what we like. We see max G's about .710 into the run which is typical for us. My main concern is where the wheelspeed is at .5 into the run. Show me that number and I can tell you if it was a good run or not. FWIW the weather for that run was a baro of 29.3, 61% and 90 grains wasn't a bad run at 7.90/174 but it could have been better with a stator swap or more gear that run. But we have to make a judgement call on that stuff early as its just two of us working on the car.