I was watching a trainer recently telling her student about how to approach the half pirouette on the diagonal, seen in Fourth and Prix St. Georges. She suggested, since this rider's pirouettes tended to be a tad large, that she start wide on the diagonal coming out of the corner so that the pirouette would finish on the correct line (and thus maybe fool the judge).
Sorry, but this is kind of like waving a red flag at the judge that the rider fears the pirouette is going to be too big, so the judge is actually watching for it. Even if the pirouette turns out to be the right size, the rider might lose a half point or more for not being on the diagonal in the first place.
I see a lot of riders do this, but I don't always comment on it, mostly because there is so much else going on at that point and the poor scribe wouldn't have the time and space. I fret a bit that maybe this leaves the wrong impression, that the rider might think it's okay to actually not perform that pirouette on the diagonal line. Sometimes I go back and add something further about it at the bottom of the test if I have time.
So, what is a good showmanship strategy if the rider is concerned about the size of the pirouette? First, try to show a clear transition between the collected canter and the "pirouette canter," the increased collection that should precede the turn. That's half the battle and makes a good initial impression. Even if the horse can't sustain that balance and steps out a bit, it still is the right way to start.
If the pirouette ends up on the large size, after the turn the rider should head back on a steeper angle that will put her in the correct position as she approaches the corner letter. This adjustment to a different diagonal line after the turn is less apparent to the judge than starting that way in the first place. And, if the pirouette turns out just fine, starting and ending on the correct diagonal line, then there won't be any unnecessary loss of points to begin with.
But, gosh darn, the half pirouettes aren't easy, a real moment of truth. I can easily sympathize with a rider who can't yet quite pull them off.