This app is a good idea. Using in the UK our spirits are 1.5L magnum bottles which sit “upside down” on optics. This makes using the app harder because you don’t have the 1.5L size but also because I have to remove every bottle from the wall to turn upright, rather than taking the measure with the bottle upside down.
Chris Owen commented
The bottle doesn't need to be upright to measure the contents. The app doesn't try to guess how the shape of the bottle affects the volume of liquid it contains. It just assumes that the cross-sectional area is uniform.
Of course, that's not even remotely true for some bottles. So one solution is to create your own scale and paste it as an image next to your bottle on the photo. Then the stocktaker aligns the contents level line with the scale instead of the actual liquid level.
Regarding the actual contents (1.5L) just set a value of 1500ml.