You'll need 3 extra bottles. Assuming for the sake of simplicity that you start with 20ml of Na2 at the 20% delivered strength. Put 10 ml of it along with 10ml of distilled water in a spare bottle. That is your 10% solution. Take 10ml of that and add 10ml of water in the next bottle. That is your 5% solution. Take 10ml of the 5% and add 10ml of water. That is your 2.5% solution. You are now done.