In joint work with Ric Wade and Andrew Sale, we give an algorithm for the kind of calculation described in the title. The main tool in this is the short exact sequence for relative outer automorphism groups of RAAGs using restriction maps, from previous work of Day–Wade. The new ideas in this work include the application of a theorem of Bieri on rational vcd, and a computation of the vcd of certain Fouxe–Rabinovitch groups using the Guirardel–Levitt outer space.