His total financial assets are 282,000, and since he is already so close to 70 I would assume he isn't going to save a lot more before then, and the pile won't grow very much more in the market by then either. At a 4% safe withdrawal rate, he can only take out about $11,000 a year to augment his SS (he doesn't have to follow the 4% SWR of course, but we need to make some starting assumption if we want to get an answer). Even if all of that were taxable income (it wouldn't be, as some would presumably come from savings or from principal of I-bonds) it would be below the standard deduction, so the tax rate on it would be zero.

So even if you wanted to take the tax rate into account, being zero you still wouldn't need to take it into account. The advice to worry about this would mostly only apply to wealthier people.

By my calculation, he would need to be taking about $20,000 in taxable income out a year before he would start to pay federal income taxes. At that point, the marginal rate would be 18.5%. (The taxable dollars themselves would be taxed at 10%, plus each one would cause 85 cents of SS benefits to move from untaxed to taxed.)

That is for federal taxes. My understanding for MI, $11000 in taxable withdrawals would put him just past the point where it exceeds the MI standard deduction for married, and so would start being taxed at 4.25%. But the legislature is still in session, so...