http://www.stowiki.org/Damage_resistance

this is how i determin when stacking more resistance is obsolete. and in my opinion thats around 35 damage resistance. Now you could say, thats exactly 2 neutronium consoles...

...BUT, through skills, atleast i start at around 18.6% resistance (6 points in starship hull plating = 23 dmg resi magn). So adding only one neutronium mk xi (17,5 dmg resi magn) will get me to my desired 28.5% which is around 40.5 damage resistance magnitude. Thats around the point where the curve starts to flatten too much, and adding a resistance console onto that becomes a waste.

anything that gets you above 40 damage resistance magnitude (cosnole + skills) is argueable because it still only effects hull dmg and not shield dmg, especially when you sacrifice a tactical console slot for it, which is the main point of critique actually.
The only ship i would aim for 75 resistance magnitude is in an hull tanking cruiser (which is senseless anyway), but thats where the curve falls off drastically, but still well in reach for a monotanium alloy console (40 resi magn) and kinetic resistance.