Difference between wounds suffered (5) and the toughness bonus (3) = 2 critical.
So lasgun (1D10+3) roll 5 +3 (8 damage); hits the poor citizen in da face. Said citizen has 3TB (because normal cititzen), 8-3 = 5 wounds.
From his starting 10 wounds, the guys drop to 5. But since he lost more wounds than his TB, the difference is also critical damage (so 2).
Problem of my way: there are rarely very high critical damage that blows up legs and such without being an insta-gib also, unless there is a righteous fury (+1D5 critical) involved.
So for another example, Rick the guardsman has a light carapace armour (5 all), 5 toughness bonus and 17 wounds (what a tank)
He gets it by a bolter and suffers max damage (15 pen 4). He loses 9 wounds, which is 4 higher than his TB, making him suffer 4 critical damage (+1 because bolter is damage type X). With this, it is also a righteous fury, adding +1D5 to an already dangerous 5 critical to the head...there are a lot of chances that RIck dies even if he still have 8 wounds left.