> I use a nominal 5V from a 7805 on lots of designs for VCC with
> a 5.12v reference from a TL431

Steve, I like my maths simple too. My choice is between 5.3V Vcc
and 5.12V reference or 5.12Vcc and using that Vcc as the reference.
As you say, can be had with a TL431, or a jacked-up 78L05 does
as well

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