Wiktionary
right as rain
a. 1 (context simile colloquial English) Very good; healthy. 2 (context simile colloquial English) correct; factually accurate.
Wikipedia
Right as Rain
Right as Rain is a 2001 crime novel by George Pelecanos. It is set in Washington DC and focuses on private investigator Derek Strange and his new partner Terry Quinn. It is the first novel to involve the characters and is followed by Hell to Pay (2002), Soul Circus (2003) and Hard Revolution (2004).
Usage examples of "right as rain".
That tells me that David Brock, while no longer right, is, in fact, right as rain.
Now little Colleen's engaged to John Barrington-Stone, she'll be right as rain.
Here, now, wash the aspirin down with this, and you'll be right as rain in no time.