403b match based upon % of Total Salary

Does anyone have a match set up as a % of ET - Employee Total Salary? We have a small number of employees that are eligible for a 457f plan and contribute 10% of their Total salary and receive a 10% match. They can also contribute to a 403b plan which has a match, but they can't get a match based on their Total Salary, it has to be Total Salary less the 10% that has already been matched by the company. If anyone has any ideas I would love to hear them - even if it means a custom calculation.