The X-Potion, if HQ, is 20% (Max 84). Your BRD probably has more than the 420 Dex needed to hit the 84 mark, so just find out what his Dex is and divide 84 by that. Then factor in the time.

For Foe, consider that a number of people I've heard do testing claim that Foe Requiem without BV will boost damage by 15%, not 10%. This may require some extra testing. Also important to consider is at what point throughout the song BV drops. You'd have to find out how long extra the Ether is giving you, how much of that extra time is BV time, and how much FR and BV FR actually boost damage by.