magic item bonus progression

[Seatbelt]

So im doing something different with magic items in my campaign. The pcs do not have a chan e to buy items. So im leveling them up as the party levels. What is an appropriate progression of +x bonus to AB or AC that scales by level?

[OldTrees1]

Here is some information (the thread has some more):
http://www.giantitp.com/forums/showpost.php?p=16416299&postcount=21