If you got the same kit I did you have 20% solution. You need to make other dilutions from your "stock", to give you a range of contrasts.
I mixed 4 parts H2O to 4 parts Na2 for 10%
6 parts H2O to 2 parts Na2 for 5%
7 parts H2O to 1 part Na2 for 2.5%

I've attached a scan of a chart I made for myself from info in Christina Anderson's book "Alternative Processes Condensed".
Hope this gives you a starting point. Post some prints when you have some.