It all depends - flushing is more about a rising plane of nutrition causing more eggs to be released. I tend to put mine to the tup at cs 2.5-3 (and certainly no more) and I get good lambing percentages, usually. Lots put theirs to the tup at 3-4. It all depends what condition they started in. I don't like my sheep to get anywhere near CS4, because I believe sheep over 3 are too fat and won't lamb themselves etc, but I'm probably in the minority...