I agree that a typical coiled extension spring spec includes a pre-load for fully retracted length, so it should be in the spring description.
Until then, give the initial spring length according to what the spring length would be for zero tension if the coils did not interfere.
And remember that for large deformations the extension and compression loads are not exactly linear anyway. As the spring changes length the leverage on the coils changes with change in spring diameter so that k increases as the coil diameter decreases.
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