It's kind of personal preference. Basically, you just eyeball it. I'm not familiar with the juwel tanks, but if it has trim, you want to get the teeth at least half way covered by the trim, so you won't see the water line. The return pump will determine how high the water comes up on the teeth. If you're close to the max flow of the drain, it'll be higher up on the teeth, than if you're at half the max flow.
"What, exactly, is the internet? Basically it is a global network exchanging digitized data in such a way that any computer, anywhere, that is equipped with a device called a 'modem', can make a noise like a duck choking on a kazoo." - Dave Barry