Welcome Bananaflier,
First of all, like ALL math to need to do while you fly, guess at the answer first, then work it out to get to a ore precise answer which is in line with your guess.
As said, without seeing the chart, it's hard to offer a good answer, but you can help yourself by guessing the answer first. You have a "standard atmosphere" performance value, which you should be easily able to find for your circumstances. Will the difference in your atmospheric conditions improve or decrease your performance? By much? Or just a little? Now, you've bracketed the answer you're looking for - you won't get it wrong, just less right - continue looking at the chart, and/or, doing the math.
Generally, other than for extreme temps, a Palt of 600 feet will make little difference to performance, but a tiny decrease, so now you, know what you're looking for, a slightly longer TO distance.
If you're still stuck, post the image of the chart (600 x 800 max pixels please), and you're conditions, and we'll try to help you out...