whats the ideal weight for a woman 5'8?

I'm 5'8 and just started out here. 140 and toned is my target as well. I put on more than a few pounds while I was sick. When I got better, I was just lazy and didn't do anything to get back in shape. I have new motivation now. I want to fit into my summer wardrobe and look good in them, not like a sack tied in the middle lol. Recession and all. I know any lighter than 140 would be way too skinny for me as my face looks gaunt then. You must carry weight really well, lucky you. When around 150, I am a UK size 12. (US size 8)