Its simple ratio.

320 is 4x 80 so you start with 1 part 320 and to that add 3 parts water which now gives you the 1/4 ratio which gives you 80PPM but 4x as much as you started with.

100ml 320PPM + 300ml water = 400ml 80PPM

If you want a particular amount of 80PPM, divide the total amount you desire by 4, use this amount of 320PPM (1x) and top off with water to the amount you want (3x water) where the ratio is 1/4.

So, if you want 160ml of 80PPM, 160/4 = 40ml so use 40ml of 320PPM and 3x that (120ml) of water and you have 160ml of 80PPM, ratio: 1/(1+3) = 1/4. Its always <something>/<total>

Hopefully this makes sense.