But if you are using it simply to calculate "will this 2hko?" It should be accurate.. For your example, the second option is more useful because either way 374 is the exact amount of damage needed to 2hko blissey with leftovers. So I think if you look at it from the point of view of "to 2hko with leftovers I need to do 53.125% damage" it should work fine.. The only problem being that the leftovers recover only whole numbers.
Have a nice day.