The Eagle B-gap tool can be ordered from the SRAM service center by your local bicycle dealer. Please have your dealer contact the local SRAM service center to obtain a tool.
The tool gives a measurement of the B-gap for the B-adjust screw. You can set the proper B-gap in the interim without the tool by following instructions in the user manual- page 15.
You will want to adjust the B-adjust screw to place a 15mm gap between the large cog and upper pulley.