Do you want to do the masonry yourself?
The parapet height is usually the upper edge of the windowsill... it doesn't matter to me how the window is mounted according to the current state of the art, normally with brackets... the window is mounted at the appropriate height after the shell construction. This is specified in the execution planning by the mason and the window manufacturer.
In the kitchen, the countertop will then take the place of the windowsill material. The calculation is done the same way as for your marble bench at the neighboring window. Task by the same people. Overall, the working height is therefore equal to the parapet height, of course depending on the countertop material.