[azapa]

looking at your size and where you live I'll imagine your using a 7mm suit. Try 8 kilos to start. if you find it very hard to break the surface, or you are still heading up at 8m or so, put on 2K more. I would guess you'll need aprox 10K, but start light.

EDIT: sorry, missed a bit of info in your post. if you are not using a suit and you have a "dense" body as you say you should not use any weight to start. Then build up 1Kg at a time. No suit dives are very sensitive to lung volume also. If you find it very hard to break surface w/o weight try a 80% inhale instead of max. That should feel nice. Be careful and have a buddy watching!