% Total Damage and % ATT

let me intercede here a bit though, don't just get whatever gives you the best fractional change, cubes are expensive and in some cases it's not worth it, say you have 35% total damage...

getting 5% more TD would mean you now deal 40% damage.
getting 5% more attk means you deal 1.05*1.35=41.75% damage.

i wouldn't say that 1.75% more damage is worth spending a minimum of 1.2k nx.

You have a good point, and I couldn't have said much about it because I don't use NX and it lies outside most of my considerations. For super-heavy NX-using, DPS-crazy people, I usually suggest cubing if the nett change in DPS is 3%, because that's where their margins tend to lie. For average users it's really more like cubing for an extra line, or cubing a fresh equipment, in which case the DPS change is likely to be significantly, significantly higher, where the DPS growth potential is on the rising part of the parabola.

To actually consider the TS's question, I think you have to hit the wall first, which would usually have implied you have spent a great deal of mesoes/NX to actually have hit 3 useful lines in everything else that you could have potted; if you're just fresh-cubing stuff and you stumble onto a %DMG instead of a %ATT on top of other useful lines on the same equipment e.g. %Boss or %PDRignore, chances are you're better off cubing something else than re-rolling the same equipment.

Hadriel