The rule of thumb about Lunar/Stellar DMG% and saturation.
200 Elemental Mastery = Roughly 50% Lunar-Stellar DMG bonus.
400 Elemental Mastery = Roughly 100% Lunar-Stellar DMG bonus.
666 Elemental Mastery = Roughly 150% Lunar-Stellar DMG Bonus
800 Elemental Mastery = Roughly 175% Lunar-Stellar DMG Bonus.
1000 Elemental Mastery = Roughly 200% Lunar-Stellar DMG bonus.
When a character has 150% Lunar-Stellar DMG Bonus (through a combination of EM and direct DMG% buffs like Heart of the Furnace/Ineffa C1), they start to experience diminishing returns. 250% is when the character is severely bloated.
Take EM/DMG bonus seriously until you get to about 150% and then relax. Don't sacrifice other stats just to keep piling on the %bonus so to speak.
For example, C1 Ineffa (+50% Lunar-Charged DMG) with Fractured Halo (+40% Lunar-Charged DMG) and wearing Aubade (+60% Lunar DMG), With Columbina Buff (+50% Lunar DMG) and having 200 Elemental Mastery from artifacts (+50% Lunar DMG), plus 200 EM from Sucrose (+50% Lunar DMG) has a total of 300% in Lunar-related DMG bonuses.
Aino could stand to put Ineffa on an Elemental Mastery diet because 300% means hypersaturation. If possible, we would try to shave off the 200 EM from Ineffa's artifacts and get the stat rolls into ATK or CRIT.
I hope this short guide can help you visualize resource distribution in your teams better. Also, sorry for using Ineffa as the example, she's just the simplest character to explain the concept with. Good luck!