IMO, this figure is utterly irrelevant in most cases, it's hard to
get most types of screwdriver bit to exert enough force on the screw
to get it to fail this way.
And if you could, it's not relevant as by the time you've reached that
number, in nearly all cases the screw has gone too deep.

I managed to shear off the head of a dry-wall screw last week with a normal
screwdriver! (just wouldnt go in that fraction more I wanted, ended up using
a nail in that spot )