Try this at home. From the introduction to my “Lullaby Variations” for piano quartet.
Update: one of my statements was way wrong so I deleted it: this is not the tuning for traditional works in C,G, and D: it would be a comma flat.
The new audio example gives two ways of tuning: first as described in the preface: a mm=150 tick, a piano A, a violin A string closely in tune, then tuned down 10 cents to around A=437.5. Then an alternate method to arrive a nearly the same result: tune the viola C string first against the piano Ab a sixth above, and tune up in perfect fifths from that.
The recording uses samples, so it’s not a perfect illustration.
“Strings should tune in pure perfect fifths. Their Eb (tonic) should be played exactly in tune with the piano. The open strings will never be used in unison with the piano. In order that the open strings may be useful occasionally, the piano should be tuned to A=440, and the strings should tune to a lower A, A=437.5. There is no need to use a special tuner: between an A=440 and an A=437.5, there is an audible difference of 2.5 beats per second at the fundamental octave. And 2.5 bps is the same as metronome marking 150, and the initial tempo of the piece.