The std Nirvana came with 15g and I added 5g. Anything more and the weight started to mess with other flight characteristics of the kite. In combination with the added 5g I shortened the upper spreader to 49cm.
It might be important to note that the later versions of the Nirvana came with a copper rod insert in the spine that was 15 g. I preferred to add the 5g right at the base of the spine as opposed to removing the 15g insert and adding 20g. IMO it positioned the weight too far up the spine. If you want to remove that inserted weight it should be secured with hot glue so carefully using a blow dryer you can remove it to verify the weight.
Hope that helps.