Hi,

No idea where you have got the thigh idea from, its wrong.

Correct seat height is very simple. With the ball of your foot
over the centre of the pedal you should be able to only just
lift your self out of the saddle left and right at the bottom.

This will also correspond to not being able to this without hip
contortion with your instep/heel over the centre of the pedal,
and also being able to lock both your legs straight somewhat
forward of bottom position when coasting in the saddle.
(Leg should lock out straight down around instep/heel area.)

I find the most accurate indicator of seat height is the locked
out angle above bottom when seated, it confirms your leg
when pedalling in the seat never completely straightens.
(So therefore you won't swaying hips because its too high.)

At my age (50+) I like to move my feet (and everything else)
around and like easy riding pushing between the ball and toes
of my foot rather than through the ball, and I think that only
works if you have the seat height near the right height.

rgds, sreten.

Typically for a good seat height you cannot reach the ground
with your feet, and starting/stopping involves dismounting
and mounting the saddle, unless you are next to a kerb.