You can't get a 5dkh solution from a 4dkh solution. I'm using a 40 dkh solution and diluting it to give me 5dkh. You have to start with a soluiton that is greater than the solution you want to achieve.
For example. If I want to use 50ml of the 40 dkh solution to make a 4 dkh solution.
40 dkh/4 dkh =10 or 1/10th the strength. Multiply the volume of the 40 dkh solution by 10 (50ml x 10), that gives you 500ml. So I have to mix 450ml of H2O with 50ml of 40 dkh to give me a 4dkh solution.
Same calculation for a 5dkh
40/5=8 or one eight. If I want to use 50 ml of the 40 dkh solution I multipy 50 x 8 = 400ml. So I need to use 350ml of H2O and 50 ml of 40 dkh solution.
You really have to make a 40 dkh stock solution to work with.
I followed these instructions to make my 40 dkh solution.
http://www.plantedtank.net/forums/showthread.php?t=42429
Mike