No standards is the standard One brand’s stiff flex is another’s regular
Shaft manufacturers set unique flex and balance points for each shaft they design. They also use a variety of materials with varying thicknesses. And the way they measure the flex of their shafts is probably unique as well. It’s clear then that buying shafts off-the-shelf is a no-go.
As there is no ‘true flex’ for golf shafts, what we really need to do when setting you up is to match your shaft with your clubhead and your swing. This means hitting shots with a variety of different shafts (that could all be ‘regular’ flex), and seeing how it feels and what your ball flight is.