Atty James has given wise advice. I have several clients in your situation. They have all been sent to tax attorneys for representation and assistance in dealing with high tax liens. You will most likely save more in the compromise that you spend on the attorney.
An offer in compromise negotiation is really something you should discuss with a tax attorney. Why? You need attorney client privilege. This open forum can be viewed by anybody including Iowa revenue agents. An offer in compromise is based on a comprehensive review of your financial situation. There are no hard and fast rules. But, do not expect them to accept a very low offer. Revenue agents will look at all the financial information you share with them and use their internal formulas to estimate what you should pay. A tax attorney can help you determine a reasonable offer after reviewing your financial documents.
My comments are NOT LEGAL ADVICE. They are for informational purposes only. Actual legal advice can only be provided after you have signed an engagement letter. Answering this question does not create an attorney client relationship. Remember that without attorney client privilege you could possibly divulge information that can hurt your legal rights in the future. I am a tax attorney in Miami Florida. I can help you with your federal tax issues via a secure client portal if required.
I would advise seeking legal advice from an attorney. You say you had a request granted, but I don't think you have because typically if you make an offer and it is accepted that is based on an actual number you give them (i.e., I will pay x amount to settle, they accept, then you are both obligated). This sounds like they are asking you to make an offer. The point of an offer is to pay less than you owe, but calculating how much depends on the issues involved and your finances.
This is not legal advice nor intended to create an attorney-client relationship. The information provided here is informational in nature only. This attorney may not be licensed in the jurisdiction which you have a question about so the answer could be only general in nature. Visit Steve Zelinger's website: http://www.stevenzelinger.com/