Don't want to get into the minute details, but Sanders proposed lifting the cap on social security. So, in effect (not arguing if it is fair or not) that is a marginal tax increase for anyone making over $118K a year. What you have there is a mix of marginal and effective. The marginal increase is real, whether we agree it is a fair increase or not. That is why you are falling short of the 73%. Also, there are many municipalities & counties across the country that levy income taxes, which not included in your figure. For example, all Maryland counties have an income tax, which I believe ranges from 1.25% up to 3%.
And while those figures above are marginal rates, the effective rate will be just as large as an increase. Why? The social security increase will impact people, treating capital gains as ordinary income with increased marginal rates will increase effective rates as will further restricting deductions for those making over $250K.
Finally, the 73% is magical because it is what Diamond and Saez believed would be the optimum rate to maximize revenues on the top end while curbing top level "greed". There were many assumptions that went into that theory, including ETI. It's a theory and like all theories, some agree and some disagree. The key disagreement is that the 73% looks at it from a pure utility perspective of the whole (all Americans) and what utility means is open for debate. In any event, the 73% takes into consideration ETI, which means it stunts growth of income (to an extent) for those impacted. Thus, assuming 73% top marginal tax rate WITHOUT a change income growth (which is what I mentioned in my prior post) would lead to a failed model on Sanders side.
https://berniesanders.com/issues/strengthen-and-expand-social-security/