Folks, I've downloaded the DND Calculator, and made a few comparisons.
From what I can see, if you're to be enrolled in the part-time plan my calculations and those of the DND calculator are very close - perhaps 2% variance.
For the full-time plans, the numbers are more out of whack. My estimates of the cost for payback are much higher. As they've tried to hide the calculations on their spreadsheet (Note to people: Excel is not a good way to keep things hidden - copy and paste works and lets you un-hide things) I've had to do a bit of detective work. I think there is an error on the official DND spreadsheet ; I will try to confirm it tomorrow. If it isn't an error on the DND spreadsheet then I am pleasantly surprised - the savings is significant. However, I'm pretty sure I know what error they have made.
To fill out the DND spreadsheet, you need your pensionable earnings for each year, along with the number of days you worked. That information goes on the "Pt I1 Election" tab. (I'm not sure why there is a separate "PILL" column). You also have to fill out the tombstone data at the top of the form - your enrolment date, the "last calendar day" of your year of enrolment (for example, I enrolled in October 1989, so the last calendar day was 31 December 1989); and at the bottom, your birth date. The other data would only be adjusted if you were not being enrolled as of March 1st, 2007.
The sheet will indicate the maximum payback, and what benefit it would provide if you paid the maximum. Enter amounts in the blue box at the bottom to see how varying your payback varies your future benefits.
If you're going to be in part I1 (part-time plan) only, that's about all you need to do. If you will be in the full-time plan, you should go to the second table, "Rollover Election". It is supposed to indicate how much additional it will cost you to buy back your service in the full time plan. However - one big caveat - it will show the amount to pay back as "Zero" unless you have entered an amount on the previous tab for "Your Buy Back". I won't tell you how long it took me to figure that out.
More to follow as I look at the DND spreadsheet some more...