The standard advice seems to be wear weight that makes you neutral at 10metres. This seems pretty good advice to me as your chances of blacking out are greater at -10m to 0m depth as you go from 2 atmospheres to 1 atmosphere.
As everyone is built differently and different kinds of wetsuit have different bouyancy qualities it is pretty difficult to say how much weight you need. Experiment and slowly build up the weight until you are neutral at 10metres.
Whatever weight you end up with another important consideration is the type of belt you use and how the weights are spread along the belt. Many freedivers swear by expanding, rubber type belts which you put on mega tight on the surface and they stay tight as your body and wetsuit compress on the way down. Net result, the belt should stay at your waist no matter how deep you go.
The other type of belt (and the one I use) is a simple nylon belt, non-expandable. These belts will slide to your armpits when you dive in a head down position no matter how tight you cinch it down on the surface. I personally like this set-up as your centre of gravity moves towards your head and this feels like a very natural gliding position.
As for the spread of your weight, thats pretty much a matter of personal choice. I put all my weight at the back of my belt. For standard freediving it stays that way but when I spearfish I slide all the weight round to the front so it hangs by my belly. As I spend most of my time floating on the surface this keeps me really stable.
Sorry if you think this is a cop out answer to your 'how much weight' question but you really need to experiment.
Cheers, Brian
