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]