Does this cap go in series with the coil at the top, or does it go in
parallel as do the 80 and 40 meter caps?

Are you aware that you have to use a 1/4 wave lenght impedance transformer
(matching line) on 20 meters?
This is the text from the original manual:

Symptom: High SWR on 20 meter; other bands OK.
Look for: Missing matching line. Antenna not properly tuned. 20 meter
radials not present or wrong length. Consult instructions for tuning and
radial information; install matching line RG-11 75 ohm coax, 11 ft 4 in
(345.4 cm) if solid dielectric, 13 ft 6 in (411.5 cm) if foam type.


73 de oz1akn, Mogens