MOSFET Rds(on) minimum
It's possible to use Vds to detect a "huge current' such as a short circuit (and that is done) but Rds is not a good way to measure current.

It's variable from unit to unit, and it varies considerably with temperature, perhaps increasing 50% from room temperature to normal operating temperature. 

If you like guessing you can take the difference between typical and maximum on the high side and guess it might be similar on the low side. You'll probably hit three sigmas that way and maybe that's good enough if 1 in 1000 is outside.