A few basics, too large a foresight is worse than the best size, but too small a one is a serious disadvantage. The aiming mark must be contained within the foresight ring, if it wobbles out of it when you’re aiming, you need a larger foresight.

You say:

With the bell target I thought I needed to go smaller as I'm not very good at judging when it's central.
That seems to indicate that you might be watching the aiming mark? The aiming mark doesn’t move (believe it or not), it’s the foresight you must keep in focus and concentrate on. A nice well defined ring of reasonable thickness helps, not a thin one. When you look through the sights the foresight must be the thing that attracts your focus, not the aiming mark.

Now for the difficult bit, size. Everyone’s eyes are different and that can lead some variation in what people perceive as their ideal foresight diameter. Try not to buy a handful, as you will end up using just two or three. Most clubs have a few in a drawer somewhere and many shooters have a few spare that you might ask to borrow. Have a go with 4.8 at 10m and see how you get on. If it’s ok, then look for a size that gives a similar sight picture at 6yds. You have to find what’s best for you. As you progress and reduce your movement, you may find that you are able to drop a size.

Rear sight aperture of 1.0mm is ok, depending upon your distance from the sight. As a guide, the “normal “ range with an eye relief of 2-3cm is 1.0-1.2mm.

Hope this helps,
Rutty