Hi Eugene
These walls are actually spaced at about 586 Million light years. The reason for the difference from your stated figure of 420 million light years is that astronomers often quote distances in MPC/h0 where h0 = actual hubble constant / 100 km/s/Mpc. In the popular press they usually ignore this (probably because they don't understand it). The reason is that the Hubble constant is not well known.
So the stated 128 Mpc = 420 MLY is to be divided by 71/100 which is the current best estimate of the Hubble constant. result ~588 MLY.
This result is fasctinating because geologists report a cycle of period about 590 MY and Prof. of Geology at Moscow, S Afanasiev, gives the accurate period as 586.24 MY. From this the Hubble constant can be accurately calculated as 71.1 km/s/MPc.
The interesting thing is that the walls are spaced at exactly the distances that the nodes of a wave would be located for a wave oscillating at the geological cycle period. This all supports the Harmonics theory view of a wave universe. See my web site at
http://ray.tomes.biz
Regards
Ray