The proportion itself is based on expected life span, and the IRS has a great table that will show you the percentage. But, I think the problem is more complex than that, and you need a short consult with a local property / wills attorney. The will must be examined, and the details of the proposed partition.
We do not have a client/attorney relationship until you make an appointment, we discuss your case face to face, I accept a retainer, and we explictly agree to enter into representation.
Most people dont want to buy a lawsuit. But if the property is that valuable to you, consult a lawyer to help and try to acquire both interests without litigation. You are in the good position of not being a party in interest right now, and you may be able to wrap it up before becoming one in a lawsuit.
DISCLAIMER: The forgoing comment is for general educational purposes only, and is not legal advice upon which the reader may rely as the commenter has no actual knowledge of the facts of the case, has not interviewed persons or examined evidence, and has not researched the applicable law. The comment is based only on the facts provided, which are extremely limited, and may or may not be true. Complete defenses may prevent the success of any claim. Competent legal advice should always be obtained before taking any legal action or filing suit. Readers employ any information provided herein at their own risk.