I reran multidiel_rt with the as-built coating structure. The penetration depth is x₀ = 560 nm. With A = 5.6 ppm absorption on each mirror, the absorption coefficient is therefore α = 0.05 cm⁻¹.

Penetration depth x₀ is defined via E(x)/E(0) = exp[−x/(2x₀)]. Absorption coefficient is defined as α = A/(2x₀), since the effective distance traveled through the coating is 2x₀. [I belive this is the same definition that Garrett uses.]

The script for this is in the paper directory of the svn, under source files.