Gravy, a couple years ago, the top of my D. marginata snapped off, by accident.
I placed in 'water.' Within two wks, baby roots formed. I waited for roots to established before adding to soil, so left in water...changed water daily or every other day.
When roots grew 7-10", the top portion was potted in soil.

Whichever way you decide to prune, it should be done in spring, not fall.
Air-Layering is probably a better way to root instead of cutting the top and placing in soil.

Reason being, roots are formed when ready to go into fresh soil. Also, don't know if you want duplicates, but the bottom half will produce leaves.

Come to think of it, no matter how it's cut, foliage will grow from the bottom trunk. Toni

GB - cuttings and air layers strike fastest when the plants energy reserves are greatest & cultural conditions are most conducive to growth, which for you will be July & August. The plant comes easily from cuttings, so an air-layer would be extra steps, but an air layer does offer greater certainty of positive results.

Cut well below the leaves on a diagonal with a very sharp knife or a fine saw. Don't use a tool that could possibly crush tissue when you cut through the stem (like pruners - especially anvil pruners). If the stem is too woody to cut through with a knife, use a fine hacksaw & then use a sharp knife to clean of ALL tattered tissue emerging from the cambium, as these fine remnants are where rot starts.

Use a deep container for cuttings and a very fast draining medium. I don't know if you use the gritty mix, but it's excellent for cuttings. Alternately, use 100% screened & rinsed perlite, and when you pot the cutting, make sure the proximal end of the cutting is several inches above the bottom of the container. This will ensure that there is never a film of water covering the cut end - it's important that air can get to it, which is why a well-aerated medium is a big plus for cuttings, as well as our plants.

Good luck.

AL

Al, ever air-root using the girdling method vs. cutting through the stem?