item Perfection and KeepDecision confusion with "sell darkening"

[JarJarD3]

I've got two Obsidian Ring of the Zodiac rings, one has perfection value of 75 and is highlighted in stash.
Imgur: The magic of the Internet
The other has perfection value of 83 and is not highlighted but dimmed if in stash.

I wonder how item "KeepDecision" and "Perfection" are calculated as it seems that the better ring is is not highlighted as it IMHO should be.

First ring perfection is 75 and KeepDecision=LooksGood, second ring perfection is 83 KeepDecision=DefinitelyBad even though it has three perfect attributes: CHC, IAS and RCR.
Of course CDR is "poorish" but definitely bad IMO.
I would rate second ring perfection to 93 (without Fire Resistance), obviously Fire Resistance lowers perfection to 83?
And maybe "KeepDecision=LooksGood" requires that all significant attributes are over certain threshold value that must be over 25?

It would be nice to know how Perfection value and KeepDecision are calculated.
Maybe somebody can shed some light about this.

2017.12.08 10.02.15.899	ITEM 'Obsidian Ring of the Zodiac' Unique_Ring_023_p2 in:Stash p=75% LooksGood type=Ring group=ring
Attacks_Per_Second_Percent           +[0,05-0,07]=0,07    mod=1048575 desc=Attack Speed Increased by [{VALUE}|1|*100]%
Crit_Percent_Bonus_Capped            +[0,045-0,06]=0,05   mod=1048575 desc=
Power_Cooldown_Reduction_Percent_All +[0,05-0,08]=0,07    mod=1048575 desc=Reduces cooldown of all skills by [{VALUE1}*100|1|]%.
Resource_Cost_Reduction_Percent_All  +[0,05-0,08]=0,07    mod=1048575 desc=Reduces all resource costs by [{VALUE}*100|1|]%.
Resistance                           -[141-160]=157       mod=1       desc=+[{VALUE}] Fire Resistance
Item_Indestructible                  -[1-1]=1             mod=1048575 desc=Ignores Durability Loss
Item_Power_Passive                   -[1-1]=1             mod=402459  desc=

2017.12.08 10.02.20.853	ITEM 'Obsidian Ring of the Zodiac' Unique_Ring_023_p2 in:Stash p=83% DefinitelyBad type=Ring group=ring
Attacks_Per_Second_Percent           +[0,05-0,07]=0,07    mod=1048575 desc=Attack Speed Increased by [{VALUE}|1|*100]%
Crit_Percent_Bonus_Capped            +[0,045-0,06]=0,06   mod=1048575 desc=
Power_Cooldown_Reduction_Percent_All +[0,05-0,08]=0,06    mod=1048575 desc=Reduces cooldown of all skills by [{VALUE1}*100|1|]%.
Resource_Cost_Reduction_Percent_All  +[0,05-0,08]=0,08    mod=1048575 desc=Reduces all resource costs by [{VALUE}*100|1|]%.
Resistance                           -[141-160]=142       mod=1       desc=+[{VALUE}] Fire Resistance
Item_Indestructible                  -[1-1]=1             mod=1048575 desc=Ignores Durability Loss
Item_Power_Passive                   -[1-1]=1             mod=402459  desc=

[JarJarD3]

I did my math wrong.
It seems that item perfection is calculated from significant attributes using formula:

(sum of significant attributes perfection) / (count of significant attributes)

Resistance is not significant, so my rings perfections are:
first ring (66 + 66 +100 + 66) / 4 = 74
second (100 + 100 + 33 + 100) / 4 = 84

First ring result if a bit different than TH calculated value because I use integer precision and TH uses doubles.

So it remains open for me what is the level after significant attribute precision is considered worth keeping (LooksGood) and not irrelevant or even bad.