what is the cap assuming you already have 60% endurance active. and then other less damage taken effects kick in, is the cap like 80? 85%?

There is not cap, but all source of “less damage taken” are multiplicative.

Let’s say you have 60% endurance, 10% from a passive and 35% from a skill (this is a made up exemple). All of those say “less dmg taken”.

You receive a hit that should deal 100 dmg.

100x(1-0.6)x(1-0.1)x(1-0.35)

100x0.4x0.9x0.65 =23.4 dmg

Basically all (different) sources of damage reduction are multiplicitive with each other.

But there is no cap if you can stack enough.

To give you some numbers with the 60% damage reduction from endurance and another 2 random sources, lets say Red Ring of Atlaria (10% DR) and Aspect of the Boar (15% DR).

Assuming you are within your Endurance Threshold and the incoming hit is 100 damage:

100 * (1,00 - 0,60) * (1,00 - 0,10) * (1,00 - 0,15) = 30,6 damage taken

So 60% + 10% + 15% Damage Reduction is 69,4% Damage Reduction:

(1,00 - 0,60) * (1,00 - 0,10) * (1,00 - 0,15) = 0,306 (that is the damage that you are “still” taking, so the damage reduction is the inverse from that to 1,00; there is probably a mathematical term for that that I am not aware of)