Just did the test with 80 values:
- Crit damage is additive with the 50% base crit damage. 2.21 is close enough to 2.25 in my eyes
if it were multiplicative, one would expect a value of 2.625.
Just answered my own questions 
but this information might be useful for 'em damage calculators out there hehe^^
You are correct, that old post
from March '15 still holds (just tested it). Hmm thought they changed something in the meantime but I guess not then.
@Mr_Scooty Deadly Strike works as expected with the 50% base crit damage. Doubles everything.