For the little i know from daggers, i know you have a better chance of landing the skill if you attack from behind, "focus skill mastery" is very important (3rd class toogle) to land these skills and basicly its all about increasing the skill landing rate. Thats why DS+Might Mortal was banned after a while, daggers couldnt fail a hit. And a lethal blow from a dagger means instant death to the oponent.
Further more i know P.Atk. is not the most important thing for daggers, thats why most daggers use +4DEX, +1DEX and +4CON dyes. Maybe a pro dagger would care to write things in more detail :)
Hope i helped ;)