I went with 12db attenuation, as the recorders used have low noise floors.

Based on some comments in the R-07 review thread, I might try for -15db (unless your -12db has worked fine for an R-07 - I saw some comments from you in the thread).

For others stumbling on this post and wanting to know the math like me, it's dB = 20log(R2/(R1+R2), where R1 = in line resistor value and R2 = input impedance of the recorder. For a ~15dB attenuation, I would need 22kΩ resistors.