If you are single, the first $40K of taxable income is taxed at the 10% or 12% rate. That means that long term capital gains will be taxed at 0%. But if your total taxable income (including capital gains) goes over that amount, part of your gain will be taxed at 15% (instead of the 22% rate for regular income and short term capital gains). You get a $12,550 standard deduction, but part of your SS benefit might become taxable depending on the amount of your pension. So very roughly speaking, and assuming you are single, the first 20K of any capital gains you realize will probably be tax-free.
If you gain is less than that from selling the fund you are thinking about, you might sell other investments to scrape right up to the zero tax limit. However, if you sell anything at a loss, make sure that if you want to re-invest any of the proceeds in the same or similar investments you wait at least 31 days.
The Taxcaster should give you a pretty close estimate.
TaxCaster tax calculator
