I'm getting ready to do my first colder water dive. I have been just diving in a 3mil suit, but I'm going to go diving in a lake in january w/ a rented 7mil suit. In the 3mil suit the six pounds of my backplate are plenty to sink me with an empty tank. How much extra lead am I going to have to add for the extra thickness? I only have a 30lb wing. Is that going to be enough lift for the extra weight?