with wetsuit on exhale flly and you should sink until the water reaches eyelevel. on an inhale you will be higher out of the water. this is not a hard and fast rule but a first estmate
different weights for different people, also depends on what thickness suit you will be diving in and will you be needing it for scuba or not? I dive with around 12kg on scuba and thats with a 5mm suite and i weight just under 80kg