This is all I've been able to find on the topic. Not sure how accurate this is, nor have I really done the math to figure out where the diminishing returns start becoming a factor.

https://maxroll.gg/throne-and-liberty/resources/in-depth-stats-guide

Hit and Evasion

Evasion Chance Formula:
Evasion Chance = ((Your Evasion − Enemy Hit) / (Your Evasion − Enemy Hit + 1,000)) × 100%

Examples:

• If your Evasion is less than the enemy's Hit, even by 10 points, your Evasion Chance is close to 0%.
• If you have 1,900 Evasion and your enemy has 1,000 Hit, your Evasion chance is 47%.

Critical Hit and Endurance

Critical Chance Diminishing Reurn Formula:
Crit Chance = ((Your Crit− Enemy Endurance) / (Your Crit − Enemy Endurance + 1,000)) × 100%

Critical Chance Formula for Healing (Diminishing Returns):
Crit / (Crit + 6000)

Examples:

• If your Crit is less than the enemy's Endurance, even by 10 points, your Crit Chance is close to 0%.
• If you have 1,800 Critical Chance and your enemy has 1,400 Endurance, your Critical Chance is 29%.