Personally I don't approve of very small kills like 8 and 10mm for two reasons. First, as I mentioned earlier, any kill zone that is smaller than the rifle and pellet combination can consistently achieve, is too small. It has to be a test of your skill, not an examination of the manufacturing precision of a lead pellet.
Second point, if you place a very small kill at a close distance - where high mount scopes struggle - you will get some misses and there is a risk of bounce back at 8 yards.
On my home course we have three 26mm kills that would not meet the current BFTA rules. Two of them are freestyle, 37 and 41 yards, and the third is a kneeler at 19.9 yards. The 37 yarder is on the floor and is sheltered. The 41 yarder I mentioned before is tricky due to its position and the wind exposure. The kneeler is actually too easy.
The hardest lane on the day wasn't all to do with distance or reducers. The last five lanes are oriented differently from the previous part of the course, and you have to walk about 20 paces to get to them. Adjacent to the first of them there is a windbreak and when you are in position to take the shot you cannot feel any wind on your body or your face, if the wind is in its usual westerly sector. Amazingly this change of position made lots of folk forget what they had learned about the wind on the previous lanes. Only four had both targets, and more than half the AA graders double dinked.