Funny thing is I didn't know whether BotRK / HTGB had a shared cooldown, but it hit me I had never built both. To be sure, there aren't many champions for whom you'd ever consider building both. But yeah if memory serves it was 54% life steal and 24% spell vamp.
By the way, someone should really try out WotA/SotSW Karthus/Cassiopeia. You can solo baron at around 20 minutes (tested).