I doubt a lawyer will accept those terms. The lawyer usually gets 1/3 after expenses are deducted.
The above answer, and any follow up comments or emails is for informational purposes only and not meant as legal advice.
Legal malpractice cases are difficult and your lawyer fronts the expenses of the expert. I doubt a lawyer will risk paying substantial expenses under your terms. Good luck and there is something to the old adage that you get what what you pay for.
You would be surprised how many lawyers will be willing to accept those terms, but I caution you to hire someone just because they are willing to take less money. Those who are successful don't need to reduce their fees as they have more than enough work at their regular rates. However, if the case is big enough to forecast $500K or more it may be less of a risk. The best thing for you to do is hire an attorney based on experience and your comfort level.
I agree with my colleagues - you get what you pay for. Do you want the guy operating his law practice out of the back of his car, or the guy who actually has an office and staff? Do you want the guy (or woman) with 20 years of experience, or the guy (or woman) whose degree still has wet ink?
Fee agreements are worked out at the first stages of a lawyer-client relationship. There is nothing wrong with price-shopping, but with a difficult case, the attorney's conditions are likely to offer less and less flexibility than what you would consider a cut-and-dried case.
And do NOT select them because they tell you they THINK they can win the case. Select them based on your personal comfort, their competence, experience and reputation among fellow attorneys.
Finally, if three out of four attorneys have told you that your case is unlikely to succeed, be wary of the fourth one saying it is winnable.
This does not constitute legal advice or the engagement of my services as an attorney.
The legal fee for a legal malpractice case is one-third of the net recovery. No attorney would agree to the sliding fee scale that you propose.
Rather than dickering over the legal fee, you should do research to find the best legal malpractice lawyer.