I was going to talk about modelled (or constrained) stats , where you use point buy to simulate what you roll, but when I asked Invisible Castle to give me a test set to work with, the first thing it gave me was 12 8 10 15 13 14. Very funny, Invisible Castle. Very funny.
The second go came up 15 13 13 9 14 14 . Perhaps not the greatest set for my purposes, but this needs to be able to work for anything. I prefer Pathfinder, so I will use its 15 pt scheme for my example. That array works out to 22 pts. You could turn that into 14 13 13 13 12 9, which has a similar spread, or 15 13 12 12 12 9 if you think at least a 15 in one stat should be retained.
Another is 13 12 14 17 15 16, a whopping 40 pts, which could probably be reduced to 15 13 12 12 11 10 or 14 14 12 12 11 10. Or even 17 12 11 10 10 9, if again you want them to preserve their high stat.
Finally I got a low score, 11 8 13 13 13 9. I think I would turn that into 14 14 14 11 10 9.
Hmmm... I suppose this sort of system requires a bit of finesse. There is no set way to deterime how stats get changed, though the general idea is to preserve the "spread" in a reasonable way. I am pretty sure that I, as a player, would be happy to play in a game where stats were developed this way though.
Edit: I missed that you were using 28 pt buy, which rules out a Pathfinder conversion,

.That would the conversions something like:
2) 15 14 14 13 13 9 = 15 14 13 12 12 9
3) 17 16 15 14 13 12 = 17 13 12 11 10 9
4) 13 13 13 11 9 8 = 15 14 14 12 10 10