You could do it on your own, but it is definitely a much better idea to work with professionals who know this credit inside and out as it can be a doozy trying to calculate the credit if you don't have a firm grasp on finances and filling out tax forms.
The average contingency rate falls between 20-40%, with most lawyers charging around 33% to 35% of the total amount recovered in a case. The exact percentage can vary depending on the complexity of the case, the lawyer's experience, and the stage at which the case is resolved.