Box with fixed background

I use the box with fixed background as parallel effect, but it’s really hard to get it on the right pace. If I move the box up, the pictures remains on the same spot. I tried to place it without fix and then fix, it doesn’t help. The Sparkle interface doesn’t match the live preview, while scrolling.
Anybody knows how to place a fixed background exactly where you want it?

The way that works isn’t really intuitive unfortunately, you generally stretch the background image to fill, which makes it as large as the browser window. If you want a different portion of the image to show you need to pre-crop it.

Thx again, Duncan.
I cropped, but now the phone view protests. I will leave it for now.
Maybe it could be improved sometime. Yours, Pieter.