It's dependent on a few things but we usually use GO-30000 more or less for this paper. A carbon ink may need more.
Inkset (what inkset do you have?)
Paper batch (Silver Rag has changed drastically over the years)
What I suggest is print few small prints and experiment.
You can also fine-tune the GO amount by simply opening the GO curve in a text editor and changing the one (30000) number you see under "# LLK Curve" to whatever you want (a larger number is more GO, lower number is less).