Raising PPM during Add back

So I have an 8gal reservoir, My ppm is at 455, I need it to go back up to 800 or something , My plants are starting to yellow a bit, I didn't want to dump the whole res, only about 2 gallons have evaporated, So how do I go about bringing these ppm's up, I'm using the lucas formula at 0-5-10

Your leaves are yellowing because 0-5-10 has no nitrogen in it. Not a good NPK ratio for any stage of growth.

As for getting your res back to your desired PPM, use some basic algebra:

(current ppm * volume remaining + x * volume to top up) / res size = desired PPM, solve for x

I got 1835 PPM.