suspension line length for a 12’ Bourd Type sea anchor?

[Brion Toss]

Hi there,
Your skydiving connection is using a good rule of thumb. The idea is that 1.5 times the diameter gives you an acceptable angle to the load, without too much chute-collapsing lateral component. This will also apply for you, but for your application wave length is a further consideration, so you'll want enough rode to get the chute one wavelength away. That way you'll avoid shock load and no-load moments.
Fair leads,
Brion Toss