You can't get an object to float in the middle of the carboy without some kind of active boyancy management. A passive object will either float, from almost all above the beer surface, to until the entire object is just below the surface. Increasing the weight further will cause the object to sink, and continue to sink until it hits the bottom.
The exception is if you could obtain stratification in the wort, with higher density wort at the bottom. Then it would be possible to have the hops float in the middle if they were matched to the same density as the lower stratification layer. They would sink through the upper layer, which is lower density, but be supported by the lower, higher density layer.
This is all speculation, and IMHO, don't think the actual improvement in flavor extraction, if any, would be worth the effort. It may be simpler to use some thin stainless steel fishing twine tied to the weighted bag so it sinks and is held at the desired depth by the twine. The twine is thin enough to pass through the side of the stopper, or could be fed through the airlock.
EDIT: To directly answer the question about how much weight to add, you need to add the difference between the total weight of the bag and the hops, and the weight of the wort that occupies the same volume. In practice, this will be difficult to get right on every try. An alternative is to add more than enough weight to weigh down the hop bag, and then find a way of increasing boyancy, such as a small inflatable balloon on the end of a long thin hose (e.g. 4mm silicone aquarium hose.) Once the hop bag is submersed and the hops fully rehydrated, you can then get them to float by inflating the balloon with air. Include a suitable valve and the air will be maintained.
Although again, lots of effort for small gain, when a simple string or wire will do the trick to suspend the hop bag in the wort.