Green’s Dictionary of Slang

about right adj.¹

correct.

1850		F.E. Smedley Frank Fairlegh (1878) 40: Ya’as, you’re about right there, Mr. Lawless; you’re down to every move, I see, as usual.
1872		Schele De Vere Americanisms 577: About right, are called those things and acts which are judged to be very nearly right. ‘Well, that’ll be about right, I reckon, but I think you might have done better, Pete.’ (J. P. Kennedy, Horse Shoe Robinson).
1883		H. Smart Hard Lines II 251: ‘I am afraid your schemes went a little awry yesterday,’ observed Mrs. Daventry to her husband [...] ‘You’re about right; they did.’.
1913		Day Book (Chicago) 7 Apr. 14/1: Young Wife (proudly) —Alfred always says there’s no cooking like mine. Uncle Crusty (with a disdainful smile) — Does he? Well, he’s about right there, poor chap.
1919		N.Y. Tribune 15 June 51/2: If you guess it’s the lague of nations he is objecting to, you’ve hit it about right.
1986		R. Beckett Dinkum Aussie Dict. 3: About right: Means that the statement/fact is absolutely correct.
1991		J.T. Pickle Aus.-Amer. Dict. 2: ABOUT RIGHT: The whole bloody truth. Just about as absobloodylutely right as possible.