I was always told that 37kg was the weight to aim for. I never had a problem getting the pure Dorset Downs to 37kg in time for Easter market, the problem was not getting them too much heavier. One particular single ewe lamb was an absolute thumper and when she was unloaded, the chap said that he would put her in the show sale. I looked around later (Only time I stayed to watch them go through), she didn't even place as Suffolks won all the awards. They also happened to be nearly 2ft taller as well, but in my eyes, you don't eat all the leg bone!
However, saying that, the crosses were slightly harder to get there, they took longer. But I still got the Spring Lamb price, which back when I was getting it, was nearly £3/Kg so I was quite chuffed. But the price dropped considerably several years ago and hasn't really come back the same, I don't think.
Not only that, but I have noticed the weights at the auctions have changed. Shrewsbury take lower weight spring lamb so for me that did help.
Another thing about the scales, BE CAREFUL!! Our scales, I have tested time and again with bags of corn, with mineral blocks etc, so in my eye they are ACCURATE!!
I find it very curious then when I have weighed a lamb before loading and when its weighed at auction within the next hour, its lost anything upto 5 kg in weight! I passed comment once at the auction (not Shrewsbury) about the scales not being accurate. Lets just say I got a few evil eyes looking at me.
I've also weighed them before the "chop" and when you compare that to the box weight, it kind of confirms my suspicion that my scales are more on the accurate side than the auctions.
Our scales are the older crate mechanical type and were originally purchased for pigs and everything is in pounds. (I prefer Kg for some things).