I have a WTB FX23 disc wheelset on my utility e-bike, 559 rims, 32 spoke, SRAM 306 hubs front and rear. Yesterday I found a broken spoke on the rear wheel. Apparently these aren't sold anymore, but they still show up all over the internet. However I can't find ANY information on spoke length or a way to calculate the spoke length. The FX23 rims don't show up in any of the online spoke length calculators, and I can't find any ERD information anywhere, not on WTB's website or anywhere else.

The spoke broke somewhere above the hook that goes into the hub, so I'm guessing I can't use the broken spoke to determine what spoke length I need, right? And I can't use the "2 spokes through opposing rim holes" method to determine ERD in a wheel that's already laced up, right? Any other thoughts on determining what spoke length I need?