If your client qualifies for an ETA Offer, how do you determine how much to Offer?

Sometimes it becomes obvious, as the numbers will indicate how much to Offer. But in other cases, cases where the taxpayer really should pay close to nothing, It is a judgment call and then a negotiation. I will give you two examples:
I had a doctor that owed $450,000 to the IRS and had a medical issue that forced him into retirement. Shortly thereafter his wife passed away without having done her estate planning and he was set to inherit $500,000 from her, hence he could full-pay the IRS. We filed an Offer based upon the fact that he was 72 years old and has a life expectancy on the IRS tables of 87 years, so 15 years remaining. His income – social security and pension – left him $1,000 short every month against his allowable expenses. We filed an ETA Offer of $320,000, calculated as the $500,000 inheritance less 15 years x 12 months per year x $1,000 short fall every month. Meaning the IRS could take $320,000 of the $500,000 and would leave him with the balance of $180,000 to live on. We assumed whatever meager interest the bank would give him would be offset by inflation. At 72 years old his money would be kept in bank or certificates of deposit because he could not afford to risk it in the stock market. The IRS accepted the Offer.
In that example with the doctor it was obvious, based upon the numbers, what the offered amount should be. In some cases though it is not obvious.
I had a former Wall Street executive fall and break his neck, becoming a quadriplegic for life, restricted to his wheel chair. At the time he owed the IRS $145,000. He had $300,000 in an IRA account, with some social security disability for income. We submitted an ETA Offer for $1,000.
Why the $1,000? Because I believed he could ill afford anything and I made it clear to the IRS that he needed every dime to cover his living expenses, and even then his IRA would fall far short of his actual need. In the end the IRS accepted $5,000 to settle the tax debt.