Thank you @jairogut! It seemed like Cd would keep falling but I now see it is holding steady at 3.22 for the run with a grid of 20 layers, 0.3 thick and 20% growth. I’m happy enough now with the residuals at 6k iterations.
I’ve done more runs but the table above says enough I think. 20 layers seems to have captured Cd better than 10, though the difference in 20-50% growth rates for the two runs with 20 layers did not seem to impact the results. 10 layers at 20% growth produced a similar plot of the residuals up to 4k iterations, but the grid may not have been big enough and Cd is too high. So to check Y+ in a globally laminar flow also requires a refined and large enough grid which becomes more coarse away from the wall.