Planning sale of investments for capital gains tax calculations

[MrCLewis]

I have some taxable investments for which I would like to plan a sale. How do I go about determining how much monetarily I can sell and still stay in the 0% CG tax rate?  The investments are all long term. I need to know how to figure how much to sell and be able to stay under the taxable $78,750 level for filing MFJ.  I am retired on SS and my wife has earned income.  I want to figure the difference between those incomes and the $78,750 level to determine this sale. Articles, that I've read, seem to say that there are some caveats that prevent it from being a simple math subtraction. Thanks for any insights. 

[Bsch4477]

The unknown is what portion of your social security will be taxed and you can find that on the calculator below.  Adding that taxable amount, if any,  to your wife's earned income as well as any other income you have such as interest and dividends will give you your AGI.  Assuming you do not itemize, subtract your standard deduction of $24,400 and that is your taxable income assuming you have no adjustments.  The difference between that and the $78,750 is the amount of the long term capital gain you can have to stay below the zero capital gain threshold.

https://www.calcxml.com/calculators/how-much-of-my-social-security-benefit-may-be-taxed