If the enemy bottom lane champs are both squishies then you might go for a straight up lethality build ( darktharr and yoummu's), maybe the build shown above if the top and support are tanks with the rest being somewhat squishy champs. If there are more tanks than damage dealers ( mid and adc) then lethality wouldn't be as effective because you have to get through the tank line up who stack armor which makes lethality ineffective.
If you need the quick answer, try this out...
Building both Crit items and Lethality items is what most Miss Fortunes do, and it's very mathematically sound.