Welcome to the LOTRO Ratings Calculator!
This tool can be used in multiple ways and it works on any level:
Basic functionality:
1) calculate rating to percentage: number columns 1 & 2. enter rating in 1 and percentage shows in 2.
2) calculate percentage to rating: number columns 3 & 4. enter percentage in 3 and rating shows in 4.
3) shows a graph of the rating to percentage formula for visual information with cap percentage and cap rating.
Planning:
If you see the entered rating in number column 1 as your 'current' rating and the percentage in column 3 as your 'target' percentage (with target rating in column 4) then column 5 shows the rating difference which can be negative (red, below target) or positive (blue, above target).
In the graph the current amount shows as green color, the (negative) amount under target as red, the (positive) amount above target as blue and any remainder to cap as grey.
Advanced, simulation:
Here it gets tricky. The basic information like in the character window ingame provides only information 'onlevel' and it doesn't show rating penetrations from mobs (an effect they have in all t2 and higher instance/raid tiers and also in Anvil T1).
If you want to calculate rating and percentages for real situations / fights then you have to take into account the level differences and penetration values. Sadly, not much information is given about this from official sources.
For level we can distinquise:
 character level
 mob level
 calculation level
Calculation level depends on the percentage stat. Not much is known about this. Is it equal to character level or the mob's level? I only know for sure that for mitigation percentage stats this is the highest of those two. For mitigations this means that:
1) if your character level is lower than the mob's level then you need to set calculation level (menu bar: shield) to the mobs level.
2) if your character level is equal or higher than the mob's level then you can keep your character's level for calculation level.
Rating penetration effects are always calculated from the mob's level. In regarding to the penetrations menu  the calculator doesn't provide a means to set mob level directly, but the menu gives you some options:
1) no penetrations: Landscape, T1.
2) onlevel fights, so character level = mob level = calculation level: Tier 2 or Tier 3(and higher).
3) non scalable raids: Rift, ToO, Throne, Abyss, Anvil. for these the mob's level for penetrations is shown in the menu (like 108 for Throne). This mob level is based on the highest boss in those raids.
So for ToO T2 Saruman lvl 78 as example:
1) decide which percentage stat you want to calculate with.
2) if it's mitigations then: are you higher in level than lvl78? > use your own level. are you lower in level? > use lvl78 for the level setting.
3) just use your character level for the other percentage stats..(?)
4) for penetration you need mob level 78, so you choose the ToO option from the penetation menu.
Example: Glimmerdeep T4 lvl 120 mobs:
1) decide which percentage stat you want to calculate with.
2) if it's mitigations then: are you higher in level than lvl120? > use your own level. are you lower in level? > use lvl120 for the level setting.
3) just use your character level for the other percentage stats..(?)
4) for penetrations, the only option is to choose onlevel Tier 3. so you can't calculate those correctly if you're not 120 in this situation.
Note:
Rating penetrations affect the cap ratings, because you need to compensate for them to reach the same percentages.
Calculations work like:
1) rating to percentage  penetration value is added (negative value) to current rating and from there a percentage is calculated.
2) percentage to rating  rating is calculated from percentage and from this rating the penetration value is substracted (results in addition).
3) cap information  cap rating information shows with penetration rating compensation.
Select your class:

Beorning

Brawler

Burglar

Captain

Champion

Guardian

Hunter

Loremaster

Minstrel

Runekeeper

Warden
Select your level:
Select penetration setting:

Landscape
No penetrations: landscape, T1

Onlevel T2
Tier 2(hidden): all T2, no skirms

Onlevel T35
Tier 3(hidden): all T35, no skirms

Rift T2
Tier 2(hidden): mob level 54

Rift T3
Tier 3(hidden): mob level 54

ToO T2
Tier 2(hidden): mob level 78

Throne T2
Tier 2(hidden): mob level 108

Abyss T2
Enhancement III: mob level 115

Anvil T13
Enhancement III: mob level 120
Select percentage graph:

Critical Hit Chance

Devastating Hit Chance

Critical/Devastating Hit Magnitude

Finesse

Physical Damage

Tactical Damage

Outgoing Healing

Resistance

Critical Defence

Incoming Healing

Block Chance

Partial Block Chance

Partial Block Mitigation

Parry Chance

Partial Parry Chance

Partial Parry Mitigation

Evade Chance

Partial Evade Chance

Partial Evade Mitigation

Physical Mitigation

Orccraft/FellWrought Mitigation

Tactical Mitigation