Hey. Welcome!
1) Why aren't you maxing your IRA? Even just throwing the dividends from your taxable account into it or something I can't read, nevermind this =)
2) Go to the health exchange website and get a quote for 50 year old you, then for 51 year old you, etc. and put it in your spreadsheet, modeling out each year until Medicare. Then make a best guess at Medicare for the years after that by looking at the cost of Medicare. For both of these scenarios, model in paying some or all of the deductible + costs for things not covered by insurance that might drive up costs anyway (e.g. you get arthritis that makes it less pleasant to cook so you order more food or something more serious) * probability of them happening in a given year * your risk tolerance. Determine your best guess plan for end of life care, noting that something as simple as mentally designating 10k invested as for end of life care today might allow it to grow to 160k when you're 90 and cover at least some of this in addition to your regular withdrawals and stache burn down.
3) What does social security estimate your benefit to be, if any? Now addressed above, and banking this for medical would pretty well cover that most likely =)
4) Your current expenses are 18k. Is 18k + health insurance your expected expense in retirement? Projected at 24k after retirement.
Updating, thanks for adding the info! At these numbers, as long as you're flexible/trust the numbers if we do have a downturn, you're in phenomenal shape. For me the only risks are if your expenses are inaccurate now or in the future. If you develop a caviar habit you can just go back to work, and if expenses go higher because of a sad thing in the next 10 years that's bad enough that you can't work, well, best retire ASAP and enjoy the time as well as you can then! You've earned it, and I think these numbers scream "retire whenever you want" =)