Translate the percentages into fixed amounts of money that is on a decreasing pattern based on their job classes or position titles. This way the actual increments of the higher positions with bigger salaries will always be higher than those with lower positions (even if they both get high appraisal ratings).
If you use percentage, then the ones with higher salary get a higher growth.
It makes sense since the person in higher salary bracket presumably contributed more or has more critical skill sets.
So it's a fair deal.
If you feel,that a particular employee deserves a higher amount, you can always deviate from the fixed increment by taking approval of the concerned manager and the business head and explaining why you want a particular employee to get more. Getting into a complex mathematical model for this its self is not worth while. Unless ofcourse, you are dealing with increments for say 1000 people across locations.
Detach performance appraisal and salary hike
Let's assume a scenario where all your employees are high performers and everybody deserve 100% or 200% hike - but is that viable to offer?
So actual factors affecting salary hike are
1. Cash flow
2. Decision of management in terms of parking how much money for hike, profit etc.
3. Demand and supply of resources
4. Skill complexity, learning curve, market rate etc.
5. Nature of business, operational model
6. Whether the company has enough cash, funding etc.
7. HR Philosophy and values