want to fast again

so i have been eating well and everything.
and i want to fast again to just lose a little more. i want to get under 140 and fast. (no pun intended)
i just HAVE to see the scale dip under it.
which wouldnt make me unhealthy thin. just thinner than i am now.
i can go as low as 127 for my height and frame.
so at what point does it become abusive to yourself to fast for losing weight?