I
would have underestimated
you
would have underestimated
he / she / it
would have underestimated
we
would have underestimated
you
would have underestimated
they
would have underestimated
I
would have been underestimating
you
would have been underestimating
he / she / it
would have been underestimating
we
would have been underestimating
you
would have been underestimating
they
would have been underestimating