I'm still thinking whether or not there's one...I first thought low% would be getting all the Photo OPs (literally all), but after trying it out a few times sometimes it's simply impossible as an opponent might beat you to it.

Low% could be to complete the Gulch level as it's not mandatory, in any% you can just commit suicide and carry on to Nuke York. But then again it wouldn't be a significant difference to any%, so maybe there's no valid low%. Alternatively you could be forced to get just one Photo OP per level, so you'd still have a chance.

So...still under works.