thanks for posting the calculator. I'll give it a try in a day or so.
I have got mine working after a little finger trouble on my part
and also found what I think are a couple of minor errors in the build guide/testing section.
The building was straight forward and I followed your build guide to the letter but double checked against my own BOM at the same time.
I found that IC8, the Notch filter 5532 amp (both sections of the 40dB post filter amp) has a d.c. offset of about 1.5 volts and is not a about 10mV volts as suggested in the guide. I tried some cherry picking on my Signetics 5532 and that was the lowest I could get it. So I put a LM4562 in instead which brought it down to a few 10s of mV. I might have a play with some alternative low noise FET opamps in that position.
I am out for a few days so I will post some photos for the scrap book when I get back.