theThe robust scale is normally outputedoutput by the routine you used to estimate the MM. For example, in R:
library(robustbase)
data(coleman)
set.seed(0)
RlmST <- lmrob(log.light ~ log.Te, data = starsCYG)
RlmST$scale
There is no explicit formula to compute it: it's the result of an iterative scheme.