^^ You're referring to UKAHFT rules. HFT is shot to other rules in some parts of the country; down here in the South West we shoot to the SWEFTA rules which do allow you to adjust the focus for each target but only before you have raised the scope to your eye. In other words, you have to estimate the distance using your mark 1 eyeball, and then you can adjust the focus to suit your estimate. If you make a mistake or forget, too bad, once the scope has been looked through at that target, you can't change it until the next target.
Why do we allow this in SWEFTA? Because our course setting rules are different, too, and we allow HFT targets out to 55 yards instead of limiting to 45 yards. It's too much to ask eyes to compensate at that distance and still get a sharp enough view of the target. Moreover, it allows you to get a sharp image of close targets which often have small kills. But as I said earlier, if you forget to adjust, you have to live with the poor image.
You may be surprised to find how quickly you can get used to estimating a distance with some degree of accuracy, without needing mildots or bracketing etc.