Just had FormulaOne Pinnacle15 (Ceramic) installed. It is rated at 18%.
It is a perfect match to my eyes. I can see absolutely no difference to the rear in any light or angle.
I have had better luck with the match by going with ceramic tint as it tends to not have as much reflection and therefore matches the factory tinted glass better.
I do not know the brand but had 20% on my previous SRT and not only was it more reflective (metal alloy tint) but to me it was noticeably lighter. It was supposedly a very high end brand but to be honest I believe the place that did them were a bit shady so not so sure.
I would say, go to a highly reputable tint shop, go with a mid to high level 18% - 20% and you should be good to go as far as a match to the darkness. For me the Ceramic is worth the extra money to also match the reflective qualities but I may be a bit more picky than others (or it could have all been in my head too).