damage affected by Weaken & Effective Mythic is calculated before the check for Critical Hit.
Deadly Strike is % chance on Critical Hit to do 2x Critical Damage. if you only had a +10% Deadly Strike on your Items:
90% of Attacks would do 1x Normal Damage.
9% of Attacks would do 1.5x Critical Damage.
1% of Attacks would do 3x Deadly Strike Damage.
if you have +60% Crit Hit & +60% Deadly Strike with +350% Crit DMG:
40% of Attacks would do 1x Normal Damage.
24% of Attacks would do 5x Critical Damage.
36% of Attacks would do 10x Deadly Strike.
Fire, Poison, & Bleed DoT’s have a chance to cause Crit Hits & Deadly Strikes.
Arcane Orbit has a chance to cause Crit Hits & Deadly Strikes.
hope this helps.