It is one of the common complaints that we hear when we suggest that OKRs and Reviews should be separated.
On paper, the formula such as Salary = f (salary base * % of goals achieved) might seem objective, but in reality, it is not. It depends on the highly subjective notion of how difficult the original targets were. Such formula also depends on the negotiating skills of employees to reduce their goals.
Overall, it's almost impossible to create an objective performance review process. It always will be highly subjective. Just accept it.
Our tip is to make the performance review process transparent. It would build trust in your team that nothing is hidden, and would make employees feel like the process is more objective and trustworthy.
Check out Google compensation process in the book "Work Rules!
" for inspiration to develop your own.