Crepe Myrtles are multi-stemmed deciduous trees. Your tree will grow to about 30 feet tall - one of the tallest cultivars. How high is too high? They bloom on new wood. The best time to prune is before it breaks bud in the Spring. Now is a good time to prune.

Start by pruning out dead & broken branches. Then prune any branches rubbing against each other or that criss-cross. Then prune out branches growing into the center of the canopy - keeping it open and airy and letting the sun in. This will prevent downy mildew.

First, how high is too high?
To keep it around 6 meters tall, to Bathgate's instructions, add selectively pruning the longest branches back to shorter side branches, at least 1/3 the diameter of the branch that you are cutting. Over time, that will result in an increase in branches, which will mean more thinning in the first stages of the yearly pruning. Don't leave stubs, which will result in unsightly "brooms".
The only way to keep it below 3 meters is to cut it back to a 30 cm stump every spring, as if it were a giant perennial. That will delay blooming until late summer, and lose the beauty of the smooth bark.

yep, and you won't get it's glorious umbrella shape as it matures. There are smaller & even dwarf cultivars available which you would hardly ever need to prune.

Thanks to all 3 of you. I forgot to tell you it's autumn here in Adelaide. I've done some cutting back and thinning and will wait until all the leaves fall before cutting back further in spring. What do you think?