Use Rem, forget about em formulas.

What is REM?

font size of the root element

Equal to the computed value of ‘font-size’ on the root element.
When specified on the ‘font-size’ property of the root element, the ‘rem’ units refer to the property's initial value.

html {
  font-size: 62.5%;
}

Now you've a base size of 62.5% to go.

If 62.5% is 10px, then:

you can use 1.6rem as 16px.

and don't bother with which context should I take into account, or whatsoever em & px's problems. Now you only refer to the size of html root element.