I've seen several places that the difference between handtimed and electronic timed to be .24.... I don't buy any of that. What they may be referring to is the starter gun vs the reaction time of the runner before they actually start unlike in the 40 yard dash where the clock starts on first movement of the runner.
I posted scientific studies on here last year, or the year before, stating the +/- was more like -.05 to +.15. If you watched the combine all you have to do is look at the original handtimed numbers and compare them to the official electronic times. You will see that almost all of the players fell into the -.05 to +.15 categories.