Light Level/Volume boxes

Why is there an entry for the amount of light? It doesn't seem to have any effect on the calculations, but if it does, what is it?

What is the checkbox for "Actual volume of water in aquarium" actually doing?

These seem like unnecessary complications to an otherwise powerful simple tool.

Also, could someone double-check the phosphate calculations for monosodium phosphate? I added enough to add about 1ppm via the calculator, but the test kit showed a much larger reaction.

Thanks,

TW