Thanks, man. DUDE, if Hallelujah is the extent of your Jeff Buckley experience (which was mine for ages), you are in for a shock and a treat.
Listen to the whole of his album 'Grace' and prepare to have a new favourite singer.
I usually get told Matt Bellamy or Tom Yorke.
Those flat high notes can serve as a lesson for all of us... you have to get used to singing that high, in a live setting, through a Mic and listen back. accurately hearing your own pitching in that setting is actually quite different to just singing in your bedroom. I didn't notice I was flat until I listened back... which doesn't happen a whole lot in my bedroom.
Practice with a Mic. All I'm saying.