It's just a ratio.
If you are going to tone 8x10 in a suitable tray, for example, you'll want about 16 oz of working solution. (that's 500cc for metric folks) This isn't very critical.... If I were you, I'll take 1 oz of the selenium solution and mix in 20 oz of water.
Personally, I make 32 oz working solution as I tone 11x14 as well. 1.5 oz SEL to 30 oz H2O. This keeps for quite a while.... so store it in a bottle and you are good to go.