Hi bro, i think 1272 projection for BC. And all harmonic i think should have ab=cd, 127ab=bc, 1618ab=bc :).
with tradingview.com, i use -0.27 in fibo retracement tool to find out the projection of any leg.
And last, im new in harmonic like you and on the way to familiar with it.
like casey, lets learn together. :)