How to make "observation below water level = 0.5m" when calibration

In some water quality observations, it measures the sample 0.5 meter below water surface. So I should appoint their location correctly. In Fig1, I find that “-2 : Fixed Depth” may be used. Yet I think this “-2” means “from bottom to one specific depth”, which is shown in Case2 of Fig-2. I need the Case1 of Fig-2. What should I do?

For your case 1 you can use the option -2, and for your case 2 you can use the option -3, which is fixed elevation.