I wouldn't do it that way....
only IF you are adjusting for an incorrect starting datum, i.e. Bad Bench Mark
Otherwise, I would either A and or B
Export points and reimport, add to new surface and lower as described.
Then you can get the volume when that 0.15 is removed.
I suspect this is for a volume computation and not for a datum adjustment.
If it isn't a volume issue - it might be best to edit the original point database instead.