Armor-granted DR - w/o complications or balance issues

[nonsi]

1. Base DR = AC/2
2. Armor DR = Base DR*Number of attacks from BAB.

That will put Rogue 20 vs. Fighter 20 at DR 3 vs DR 16.
That kind of disparity will make the Unfettered Warrior character concept an obviously inferior choice and shift the meaningful difference from the character to his/her gear - the exact opposite of what I'm aiming for.

[Edit:]

Also, your formula makes Full Plate grant DR 4/― at 1st level.
I believe that this is too much.