NAB SPP, page-38

The scale back will be determined by the number (& amount) of applications. They have left the door open to accept above the $500m however I would imagine they won't take too much over this amount (maybe only if total applications not much more than limit but who knows )
Last time they did an SPP was in 2009 when trying to raise $750m and max SPP in those days was $15,000. They received applications for $2.6b so it was massively scaled back on a pro rata basis. ($15,000 Applns recd $4,343 worth)
This time they have stated it is their intention that applications will be scaled back on a pro rata basis with regard to what your existing shareholdings are at the record date(*) but they have complete discretion how to do a scale back.
(*) I imagine this is most likely way, probably also with a min allocation!
This ensures holders get relatively fair allocation. - (IE - someone with 1 share highly unlikely to get same as someone with 10,000 shares)
So my guess in response to your post - Option 4 - 'other' ( Something like - Pro rata allocation of application amount based on existing shares held (most likely using tier levels not exact numbers?)