It would seem that he had 1822 or 1823 HP at that time Garret.
(15/32) x 1822 = 854.0625
(15/32) x 1823 = 854.5312
That is if that is how it's calculated for the additional effect (the same as Spirits Within?), and I think that's the theory so far.
15/32 is of course at 300%.
854 is however divisible by 2.
854 / 2 = 427
I have seen that relic weapons tend to do either 2-3 times normal damage from several sources.
In that case, if the 300% 15/32 rule stands through for the additional effect, he could have had:
(15/32) of 911 = 427.0312
(15/32) of 912 = 427.5
(15/32) of 913 = 427.9687
And simply had a double damage proc. 911-913 HP seems more reasonable for what I've seen him wearing, especially under besieged conditions.
In the end,
My bard does 1005 Spirits Within.
I win, the end.