After many rounds of denials and appeals, my wife was awarded a lump sum Social Security disability benefit amount of approx. $70000 which she expects to receive in June this year. One-third of this amount can be attributed to each of the 3 prior years of 2016, 2017 and 2018.
In addition, she started receiving regular Social Security disability payments in Jan. of this year.

For better tax planning purposes, I would like to estimate how much of this $70K would be taxable on our 2019 return and whether this lump sum amount affects the taxable portion of her Social Security payments received in 2019.
Is there a website where I could plug in some numbers and get a rough estimate?

Finally, I would like to know if we need to amend our 2016, 2017 and 2018 returns.