I am not quite understanding your spreadsheet. But, here's how I would do it.
XTOL bag is for 5 liters. You are going to mix the whole thing into 3.8 liter. Therefore, your stock solution is missing 1.2 liter of water.
If you wish to make it 1:0 (straight), then for 3.8 liter of stock solution, you add 1.2 liter of water
If you wish to make it 1:1 then for 3.8 liter of stock solution, you add 1.2 liter of water to make it 5 liters and add 5 liters of water.
If you wish to do this for smaller quantity of solutions, you'll just scale these numbers to suit your needs.