do this late fall or early spring reduce top growth by 3/4

I would base any top-growth removal on the relative amount of root I was able to retain on the division. Pruning top-growth may be necessary for initial survival on some poorly rooted divisions, but should not be necessary on better rooted divisions. Removal of top growth slows establishment (root growth) and should only be done if required.

removing top growth as compensation for root damage is no longer accepted to be as 'correct' as it used to be. top growth should only be removed so that the plant is stable (ie: won't be knocked over by wind catching the branches).

The reason why it's not accepted to be correct is because plants will abort top growth by themselves and pour their energy into root formation. It's a survival tactic sort of like when humans get too cold. We divert our blood (warmth) to our core body, and if the temperature keeps on dropping, we divert out blood into the brain. Excuse the comparison, but if humans were to 'freeze' our extremities would be a 'small sacracrifice' for lack of a better comparison compared to saving the core body/brain. In plant, you will notice that some top growth will die. In the case of the plant, the 'core' would be the roots (which are also capable of producing new top growth). The plant will need every possible photosynthesizing leaf to create food, and those that the plant doesn't need will wilt and die.

Second, when you cut/trim top growth, you no longer have one growing point, but two. Remember there are 'dormant' or inactive buds at the nodes. Once the top growth is cut, the energy normally sent to the single top shoot will be diverted (not always) equally to the dormant buds. That's why when we pinch annuals for a bushier plant. It actually takes the plant less energy to support one central shoot rather than a number of smaller breaking buds.

But do hear, it's not that pruning in proportion to root loss is totally wrong, because it was the accepted theory for quite some time. It's just that we keep on discovering better methods and techniques that the older accepted theories just fall out of place.

Origami, I am really just trying to know if this shrub can be divided, I don't plan to trim the top growth. You seem to be knowledgeable, and I would very much like to know about dividing shrubs - like my Japanese cleyera (ternstroemia gymnanthera).

I thought your question was answered. Yes you can divide a shrub that has multiple canes, by simply splitting the crown and the roots and replanting. Late fall or early spring was suggested as the best time to do this. Al

I am forever late. But thank you Al. I was looking for a guide, my books only refer to specific shrubs.

Can I just ask a question? I've never come across a Skimmia 9 feet tall with canes. That's very impressive. The varieties of Skimmia japonica grown over here are generally branching and bushy and about 4 feet tall. They wouldn't be candidates for division, more for cuttings. This isn't a Kerria is it, rather than a Skimmia?