1 11 8 5
2 11 8 5
3 11 8 5
4 11 8 5
5 10 7 4
6 10 7 4
7 9 6 3
8 11 7 4
9 10 6 3
10 10 6 3
11 9 6 2
12 9 6 2
13 9 6 2
14 10 6 2
15 9 5 1
16 9 5 1
17 8 4 0
18 9 5 1
19 9 5 1
20 8 4 0
21 8 4 0
22 8 4 0
23 8 4 0
24 8 4 0
25 7 3 -1
26 7 3 -1
27 6 2 -2
28 8 3 -1
29 7 2 -2
30 7 2 -2
1 11 9 7
11 10 8 6
21 10 8 6
30 11 8 6
I suddenly realized that a straight set of bonus adds to defenses won't work.
Take the following example:
Same level creature to hit number against: high defense (taken every time plus starting stat 18 plus 2 class), medium defense (taken every other time plus starting stat 14 plus 1 class), low defense (never taken plus starting stat 10 plus 0 class): assuming item gain at levels 3, 8, 13, 18, 23, 28 and not using feats or race bonus and creature is Level +4 against the defense (most monsters average level +3 to level +5 except the Brute).
As can be seen, the difference between best and worse defense at level 1 is 6. At level 30, it's 9. There is no linear way to balance the two because PCs will almost always have a large delta between best and worse and will also often face higher level monsters. At high levels, some PCs will almost always get hit on a 2 and only missed on a 1 versus one or two of their defenses (with core rules).
The delta between best and worse has to be decreased for any bonus math to work.
One example of a solution:
Every defense = average round down of best defense and itself plus add +1 at levels 11, 16, 21, and 26. Non-core feats (like Epic Iron Wind) not allowed.
So it's typically going to be two defenses losing 4 over 30 levels and one defense losing 7.
I'd try to keep house rules as simple as possible, so I’m not sure that would be worth the complication over just +1 FRW at levels 5/15/25, but if I wanted to decrease the different in good and bad saves across levels, that’s how I would do it.
What if neck slot items gave twice their enhancement bonus? Would that be too much? I'm thinking not, but I can't say for sure at lower levels. Obviously I'd ditch all the +x to a defense feats if I went this route, as well as the +x item bonus to a defense AV items.
A +12 necklace may sound a bit silly at 30th level, but looking at the chart, it brings a cleric's reflex defense up from an almost auto-hit to needing 4 to hit. Will defense would go up to where a 13 is needed, which seems reasonable for someone who should have a strong will defense.
For example, Agile Armor, from AV, is often considered good, but it's in general worse than vanilla armor 1 level higher until you hit level 30 (and there's no +7 vanilla armor to take), which means that you should typically just wait a level and get the vanilla armor instead.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.