Working at a Bridal Store part time I learned that when you measure you measure this way:
Chest --- Right across the nipples. IF you are wearing a padded bra squeeze a bit tighter, if you are not and your boobies are a liitle "low" lift the bra straps just a bit so your nipples are "straight ahead" LOL
Waist --- About an inch to 2 inches above the belly button! Depending if you are long or short waisted. but your belly button is lower than your actual waist. When measuring the waist I found sometimes the tape would snugggle under a roll, especially on me, so I usually make sure the tape is ON the roll not under it.
Hips --- Straight across the biggest part of the bum, looking sideways. That is usually just below the hip bone. where the top of the tape touches the bottom of the bone.
I haven't had any gowns come back in the wrong size... yet...
It is really hard to measure yourself! Because you are not standing straight, try to get some one you are comfortable with to help you out.