Relative to Absolute Displacements


So I’ve been reading several topics on this forum, and understand that in order to calculate absolute vertical displacements using InSAR, I must subtract a reference heights from my raster. It has been explained that this height is taken from a pixel with no expected change in elevation (usually requiring prior knowledge of the study area). However, I am confused by how exactly to do this.
Assuming I found an area which had experience no vertical change, how do I get the reference height from this area? Do I need to complete a ‘phase to elevation’ process first?

Also, ABraun mentioned in a comment (Subsidence map in 3d view), that the reference height should be subtracted from the unwrapped phase value. However, wouldn’t this be subtracting height (in cm or meters) from phase (radians)? Shouldn’t I be subtracting this height value from the calculated height values?

Thanks in advance,

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