Hi Maximdim,
I may be able to help a bit as I have had the pleasure to help many students who have the same questions and issues that you describe.To address your issue w/weighting properly, it might help to know if you are diving a neoprene drysuit or not. This is important as it will help in determining if you need the additional weight to be ditchable and how much. Proper weighting means a balanced rig. To have a balanced rig, a diver needs to satisfy a couple of requirements. The diver must be able to hold a 10' stop w/no gas in their wings at the end of the dive. A diver must also be able to swim their rig up in the event of a loss of the wing at the beginning of the dive when the diver is heaviest due to gas in the cylinders. This can be satisfied by ditching any ditchable weights if necessary. Ditchable weights are necessary when diving any thick neoprene suit, whether dry or wet. This requirement to have ditchable is due to the compression of the neoprene at depth and it's resulting loss of inherent buoyancy. The extra weight is required to overcome the inherent positive buoyancy of the suit in shallow water. As the diver descends to any significant depth, the suit will lose much of it's buoyancy (and the insulation provided by the neoprene as well) and the diver must then compensate by adding gas into the wing in order to carry around the extra weight at depth. This is not optimal as it increases drag and decreases thermal protection at depth. In a shell type suit that is non variable in it's buoyancy characteristics, a diver is able to use the same amount of weight at the surface as he will need at the bottom. Theoretically, the only weight a diver should need to swim up then is the weight of the gas in the cylinders. Personally, I include my light as ditchable weight and do not need any other form ditchable weight. If I were to require ditchable, I would use it in the form of a weightbelt worn under my crotch strap due to the bad day I would have if the weight were to be accidentily seperated from me at the bottom.
I hope this helps answer some of the questions you have concerning weighting and DIR. All these things are tackled in any GUE course as they are the foundation of good diving.
dive safe!---brandon