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generally negotiations take place within a range of possibilities; a low end being medical usage with maintenance treatment tapering off over time and a high end with possible complications and increased treatment as one ages.
a settlement amount falling somewhere in between depending on the medical evidence and arguments each side can make.
You can find the social security life expectancy table here
https://www.ssa.gov/OACT/population/longevity.html
you really should not guess at your medical usage. you can request a print out of treatment paid to date from the carrier that will help provide an more realistic usage figure. concentrate on the maintenance level rather then the acute post injury recovery phase.
you also must compute a present value amount. even though interest rates are currently low the cost of the future benefit money paid upfront is not negligible. generally it cuts the amount anywhere between a third and half.
you can use a spreadsheet function or something online
http://www.claimspages.com/tools/value-of-money

there is another factor to consider. Medicare secondary payer laws allow a carrier and a worker to setup a setaside agreement with CMS approval that will stipulate how much of the settlement is to be used for medical treatment. This will allow Medicare and the worker avoid the friction over trying to get medicare to pay for treatment that the worker received settlement monies for.
more on set asides here
https://www.cms.gov/Medicare/Coordinatio...rview.html

there maybe other considerations but that should get you started.