Ann: NAB announces results of Share Purchase Plan Offer, page-78

Here's how I calculate the estimated scale back %:

315M shares held by retail shareholders at record date
21% of this applied for SPP = 66M shares held by the retail shareholders who participated.
Divide 66M by 155,000 (number of participants) = 425 shares which is the average number of shares individually held by those who participated.

NAB issuing 33M new shares to 155,000 participants = 213 shares per person
213/425 = 50%

So it will be 50% of what you held at record date, but with a minimum allotment of $2,500.
If you held $1,000 shares and applied for $30,000, you get $2,500
If you held $10,000 shares and applied for $30,000, you get $5,000

This is my assumption