Does it show on the attack rating in the % to hit, or not? I.e. you have 60% chance to hit, then use eth rune in weapon, now does it show 75% or whatever, or is it you have to calculate yourself?

The "Eth rune mod" has always been messy. Used to be bugged and no idea if it was ever fixed or when if it was but for a very long time it would not lower the enemy's defense by 25% but would instead set the enemy's defense to -25% resulting in attack rating being pretty well useless.

It won't show anywhere in the menu tho as there is too much variability. The game sets a simple comparison of your clvl + atk rating against a target enemies suggested mlvl and defense. It does not take into account other factors like area, minion, or modification. It does not compare to the exact target you are striking but rather the"encoded value of target enemy type"