How reticent are police in using "justifiable" force to kill criminals?
To the extent that the FBI's count of justifiable homicides by police underestimates the true number, the true percentages are higher. Suppose that justifiable homicides by police is three times higher than reported, that would imply that about 2.16 percent of assaults ended in justifiable homicides by police. For assaults where police were injured, about 8.2 percent of those cases would end in these justifiable homicides. People can adjust these numbers as they think best.
The problem with FBI justifiable homicides by police is no where near as bad as the numbers for justifiable homicides by civilians, but we don't really even know the size of the errors here.
To the extent that this measure of assaults doesn't include all cases where police might feel threatened, these percentages are too high.