"Form 8962 line 5 shows 401% but when I divide line 2a by line 4, I get 408%"
TurboTax (and the IRS) max out the entry on line 5 to 401%, so this is not an error.
"This is important because TTax is saying I should get a tax credit for my insurance premiums, when I am NOT below the 400% line for the cutoff."
There have been changes in this area by the IRS (see below). Again, TurboTax is doing it correctly.
Note that the 8962 instructions state (on page 1 under "Applicable federal poverty line percentages"😞
"For tax years 2023 through 2025, taxpayers with household income that exceeds 400% of the federal poverty line for their family size may be allowed a PTC."