Here's my real world calculations to clarify my statement that under 30mm would be OK in my opinion.
I measured my Summer tyres when I took them off back in November.
All of them are Continental and exactly the same model.
The nearly new rears had a circumference of 2192mm with a tread depth of 8mm.
The fronts with a tread depth of 3.5mm have a circumference of 2164mm
That's a difference of 28mm in circumference.
I intend running the fronts down to 2mm tread depth, at which point the rears will probably have worn down to around 6.5 / 7.0mm.
I firmly believe that if you have the same tyres all round then you need to
keep the tread depth difference to 5mm or less.
I now swap my winter tyres round regulary at 5k to equalize them - but I was too late to start doing that with the summer tyres so they are now too big a mismatch to swap front to rear.
So - back to my original opinion that the bigger circumference go on the rear as long as the difference is 30mm or less.
Of course everyone must do as they think best.