We send our Hebs and Shetlands off at 16 months - we are in southern Scotland. That takes us into early August and gives a couple of weeks before the next lot of males are weaned. It means that the hoggets have had the maximum amount of spring and summer grass to put on condition and to get that grassfed taste.
We have tried all sorts of different slaughter times over many years, and 16 months has emerged as the best time - max weight before there's any chance of tupping time taint (although I've only noticed taint in older tups, and many people don't taste it at all).
We don't weigh the hoggets although we do check their condition score - which actually isn't all that helpful in primitives as they tend to store their fat internally. We send them off in August anyway, then get the dead weight tickets from the slaughterhouse via butcher. From the dead weight you get a rough idea of liveweight - our Hebs are usually somewhere between 17 and 23 kgs dead weight at 16 months. The weight you get back from the butcher is less than that as bones and scraps have been trimmed off.
As slaughter charges are the same no matter what the weight of the animal, you might as well keep them until they are the max weight they will reach that year, and let them grow on the abundant grass of summer which you might not be using for anything else. You will already have noticed that primitives don't put on any weight or condition over the winter, so they will be playing catch-up just now. Were you to slaughter them now you almost might as well have sent them off in December and saved overwintering costs - but they would be about 9 - 12 kgs deadweight.