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@lae
Apparently the majority of manufacturers (this Ribble diagram explains it clearly) measure from an intersection with an imaginary line between axles, meaning that the HT and ST angles have precisely the same effect on reach (assuming bars are level with saddle).
Planet X measure ST from the BB and HT from axle line, meaning the HT has marginally less effect than the ST.
I haven't found any diagrams to suggest that the HT angle makes 'very little difference'. If your framebuilder told you this, I expect it is because he measures in a different way. But then again perhaps I am just being thick.
I was sceptical about "1 degree = 1cm" because it obviously depends on the length of the tubes, but according to Mister Casio the difference is minimal:
74 to 73 on 52cm frame = 0.7cm
74 to 73 on 64cm frame = 1.1cm