I hang mirrors professionally and think that jmm1837's reply is spot on. For a client of mine I would hold it up and make a decision in situ when the client thinks it 'looks right'. That said. I would probably start by recommending a position where the base of the mirror is close to being in line with the top of the bell jar. You want it to be high enough to not make the mantle top ornaments look cluttered. But you may also want it low enough to reflect a tall flower arrangement. Hope this helps.

They look great in that photo, Northstar! Works beautifully. Will be fine over your bench. Didn't realise how large your p really space will be. Wow! No, if you replace pendants that have staggered drops, the fittings are completely removed and new pendant fittings are installed in their place. The electrician just shortens the drops without cutting them & the extra lenth is hidden away in ceiling.

Looking at your lighting plan, it isn't not enough lights, it is where they are positioned. The three lights above island should be spaced directly above island for task lighting. Your plan has them across the kitchen, the same as other three in the middle of the kitchen. If you look at the photos that have been posted with pendant lights, they are all positioned over the island bench, not spread over ceiling. Hope your choice of lighting works well.

Direct downlights over your vanity basin will cast dark shadows down your face and make you look ten years older. You are better off with softer lights either side of the mirror at eye height. If you don't believe me then grab a hand mirror and look at your face when standing under a spotlight. Save your money and buy the cheaper 3 in one heat lamp. Don't bother with the downlights over the basin - even though your electrician will tell you that is how he always installs them. Spend what you have saved on good wall lights, then you can thank me for making you look ten years younger due to better lighting and better make-up. Best of luck Dr Retro of Dr Retro House Calls