The first question you have to ask when judging a fighters weak chin is: How could an opponent potentaily capitalize on this weak chin? In Lewis's case you would have to either be verry lucky, or be great and do something verry clever to boot.
I'd rate his chin around 7. He was starched by single punches a couple of times, and has been hurt multiple times. However he was only KOd twice, one of them premature, and he fought and got hit by a lot of big punchers without going down.
Probably 7 as it would be too good to say 8. The punch that Rahman landed would of prolly dropped many HW's. The thing with that punch is that Rahman did not catch him on the tail end of the punch,he caught him where he had no where to go thus meaning he could follow throw with the punch. The McCall shot Lewis probably did not see, also its not like he was unconscious he did get up,albeit on shaky legs. Lewis was always rockable though
Lewis' chin is underated. Yes he got ko'd by Rahman & Oliver he avenged those losses and he even toke good shots by Tua a few times but didn't go down. I'd probably give him a 7 as it wasn't above average.
Shaky. On a scale of one to ten, somewhere around a 4. Like everything about Lewis, it seems his shaky chin has gotten overrated too. Top level fighters with decent chins don't get taken out in the early rounds by guys on the level of McCall and Rahman.
And fighters with "shaky" chins do not last after taking some of the shots that Lewis took from many of the top sluggers of the modern era. I think you would do better to analyze Lewis's chin further. But I'm guessing you won't.