With 140% productivity, each green circuit plant and each cable can supply 8*1.4=11.2 red plants. So, the ratio I use is 2 cable + 1 green + 11 red.
By combining plastic+green and a cumbersome routing of the output belt I can squeeze everything between two columns of beacons:
This produces 603 red/minute, with theoretical maxium 11*5.5*1.4*60/8=635, so it doesn't quite live up to expectations. It's also a shame that there's a gap between the green and the second cable plant which adds the need for the extra beacons to supply the first cable plant. So, some optimization is possible .
It consumes half a belt of copper, a quarter belt of iron, and 1/3 belt of plastic, but there is no obvious room to pass it through so that is of limited value. I've tried working with a combined iron+copper belt to save space, but I can't find a way that removes the gap without starving either the cable or circuit plant.
blueprint
[See also red+green+blue (viewtopic.php?f=8&t=37858) and green (viewtopic.php?f=202&t=33355#p210326) racks]