Below are the assumptions made for all of my calculations or facts, based on my personal situation.
In 2017, I was in the 28% federal tax bracket. Beginning in 2018, I am in the 24% bracket. Federal income tax applies to Social Security income and IRA distribution income. For me, the first 85% of Social Security benefits are taxed.
I live in Iowa. My Iowa State income tax rate has been 8.98% for the entire period. Iowa does not collect tax on Social Security income, but treats IRA distribution income as earned income.
COLA (Cost-Of-Living Adjustment). Enacted in 1972 and first applied in 1975, this adjustment was created to ensure the purchasing power of benefits was not eroded by inflation. The annual COLA adjustment is tied to the Consumer Price Index (CPI-W). The COLA has ranged from 14.3% in 1980 to zero for the years of 2010, 2011, and 2016. The average over the 40+ years of adjustments has been 2-3%. This is tied to the rate of inflation, which has averaged about 2.46% (since 1990). For the sake of simplicity, I will not include either as they arguably cancel each other out.
Return on investment gets a little tricky. To simplify this component and for the task of projecting, I will base historic performance on Vanguard's Dividend Appreciation Admiral Fund (VDADX) as reported at the end of each month. Historic average returns for the stock market or sectors of it vary in how it is calculated. You will be able to find many different values and ranges. Agree with me or not, I am going to use the value of 7.5% which is the historic average of large cap companies since 1926 with inflation factored out.