I have looked at / analyzed the Schnupp data that Annalisa and I took last week, as well as some more Yarm data that I took this week.
I only have one set of Xarm data, but 3 sets of Yarm data. I had intended to do careful error analysis of the data, but from the 3 sets of Yarm data, the variance in the answer I get using any one of the Yarm sets is much larger than the error in a single measurement.
Using the central Yarm zero crossing, I get a Schnupp asymmetry of 3.9cm. The other 2 Yarm data points give Schnupp asymmetries of 3.7cm and 4.1cm, so I'm claiming a value of 3.9 +\- 0.2cm . This is within error of Jamie's measurement of 3.64 ± 0.32 cm (elog 4821).