Backstab window is extremely hard to find because of the extremely bad animations of the enemies.
Increase the window range and lower the damage to balance it.
Because i doubt you will fix the attack animations of the enemies, which is much more work.
(enemies upper body are twisting around during attacks and its impossible to tell where their back is. Its just hoping and guessing and waiting for the icon, while losing time and getting hit by something else)
Ive addressed this a year ago.
And maybe give dual daggers the biggest backstab damage and then single dagger and then base backstab damage on the size/weight of the weapon. As smaller the weapon as bigger the backstab damage is.
Thats proper balance.