After reloading from a quick because the second to last draugr, the DDL with the .50 caliber sniper bow (Ebony Bow + Arrows) ACTUALLY KILLED LYDIA, I decided to do a suicide attack, just to see how much damage this fierce adversary would deal. No armour - I removed my Elven Light Gauntlets, and swapped out my Leather Boots for Weightless Shoes - so I went in with intent to take full damage.
Ebony Bow + Ebony Arrows = 37 base damage.
getav marksman = 93: This yields +186% damage, since NPCs scale at +2% per skill level (vs. the player's +0.5%/slv.)
Final damage: 264.55 (it hit me from 150 max, to -114.55.)
264.55 / 37 = 7.15, the effective multiplier for the DDL's damage.
7.15/2.86 = 2.5. Since the draugr has no perks (according to checking hasperk for the archery perks,) where is this 2.5 coming from?