Is collecting my fee based on a percentage unethical?

I am a private consultant in the restaurant/hospitality industry. Right now, there is a lucrative tax credit that scores of entities are eligible for. The credit is dependent on the number of employees/wages the entity had for the quarter in question.Right now, I charge $2500 for 1-30 employees, $3500 for 31-50 employees, $6000 for 51-75 employees, and $8000 for 76-100 employees. Would charging a percentage of the credit instead be unethical?For example, I calculated a $226,000 credit that a client will receive back in the form of a refund check from the IRS. For that credit, my fee was $3,500. Colleagues are suggesting I charge 10% of the credit I calculate. That $3,500 would jump up to $22,600.My day job (where I do similar work) is charging on a per-employee basis and in a meeting we had, it was suggested that charging a percentage of the credit would be unethical...similar to tax returns. I agreed.What are your thoughts? see hubwealthy.com/wealthy