Just worked it out
100ml dilutes by 1.2% so you need to add 83.33333ml to water down by 1%
That does not work like that. The weaker the concentration gets the more water you need to add. 100ml reduces it by 1% but to reduce it by a further 1% you need to add another 150ml of water. Then to reduce it another 1% you need to add a further 250ml.
So to change 500ml of 6% to 5% you add 100ml.
The sum then changes to 600ml of 5% you now have to add 150ml to make it 4%.
The sum now changes again to 750ml of 4% to which you have to add 250ml to make it 1ltr of 3% which is the same as adding 500ml to the 6% mixture.