From Zybeks study:
Hand Time v. FAT Time Data
Analysis:
As detailed in the previous section, most variables were minimized or eliminated. The timer that measured the difference in the Hand time to the FAT time was started when the athlete lifted their hand and stopped when the coach at the 40 yard line depressed the start button on the stop watch. (These times were recorded for all athletes both 40 yard runs.)
As illustrated in Figure 3, the measured differences between the hand held and the FAT timer ranged from 0.10 seconds to 0.25 seconds. The time differences were evenly distributed between 0.10 and 0.25 seconds, with the average being 0.175 seconds.
www.zybeksports.com
This is just from their study, but in my experience it’s typically 0.24. Don’t have any studies to promote this but 10 years of experience.