Effective with DGR 54th Edition, UN2809 has been renumbered to UN3506 Mercury contained in manufactured articles. As stated towards the end of PI 869, the net quantity to be shown on the DGD is the net weight of the manufactured article, i.e., the net weight of the mercury lamp. Gross weight indication are now only with Lithium batteries under PI 965 Section IB, PI968 Section IB, and LTD QTYs which are limited to 30kg G. Net weight is the weight of the substances or articles without any packaging material.
You need, therefore, to enter the newt weight of the mercury lamps and not the net weight of mercury. However if the net weight of mercury does not exceed 5kg, as stipulated in SP A191, you may be exempted from entering 6.1 subrisk on the DGD as well as on the package. You will need to show the net weight of mercury and SP A191 on the DGD. Having said that, however, JPG-21 only addresses the label, therefore, if the net weight of mercury is equal to or less than 5kg, you will still need to enter 6.1 subrisk of the DGD and also label the package with a 6.1 label. Furthermore, please check SP A69 which shows other mercury articles that could be exempted.