"Flat" is a fiction.
Consider all the possible variables
and the idea is clearly meaningless.
Also, put your ear to the wood and
pluck various notes. Not the sound
you want, amped louder but naked
and unaltered, for your gigs, is it ?
OK, then like all the rest of us, you
play waaaaay awaaay from flat. So
very far away that center click is of
no use to find "flat" EQ. There is no
setting anywhere at all, not on any
of the knobs that sounds like when
you put your ear against the wood.
Rightly so, cuz NO ONE plays that
sound in a band.
Bassically you want an ax-and-amp
with a gig-worthy tone right around
center on all knobs [ax and amp] so
you get maximum flexibility to vary
as needed from there. Center is not
flat. It's center. And you either like
it as a good starting point of you've
got the wrong ax, amp, or both.