One of the problems of the Assassin's Death Attack was it taking three rounds and it's low DC. I didn't want to drop the three round requirement so I upped the DC.
Let's run some numbers.
10th level rogue, 20 Intelligence. Death Attack DC 25
A class with favored Fortitude has a +7 base. Figure in a modest +2 Con modifier for +9. There are a number of abilities in the revised classes that grant a bonus to saves. At 10th level the bonus would be +3, so the Fort save total would be +12.
So the above character would save on a 13 or better.
If the save DC was 1/2 Rogue level the DC would be 20, and the character would save on a 7 or better.
So it looks like full level to the DC is too powerful, while half level to the DC would be okay. Especially considering that there aren't common magic items that grant resistance bonuses to saving throws.