Hi mul211
Spring deflection for sustain could be positive/negative depends on piping configuration, operating temperature, nearby support, selected spring load as already mention by Dave Diehl.

Yes some company follow the practice to maintain sustain deflection close to zero by manual spring selection.

To fill spring data sheet, I think we should mention actual spring deflection not relative deflection as mention by you to avoid any confusion for site operation.
If sustain deflection is really large we can better specify that also in data sheet.
Max load variation still need to keep within 25%.

Regards

Habib