You can create any concentration you need in whatever quantity you desire by using "simultaneous linear equations", which is a fancy term for algebra you probably already know.
Let's say you have lots of 50 mg/dl and 150 mg/dl solutions and want 1dl of concentration C (mg/dl). Let's call the quantity of the low concentration L and the other one H.
Then 50 mg/dl x L dl + 150 mg/dl x H dl = C mg
As well, since you want 1 dl in total, L dl + H dl = 1 dl
Mathematically, that is:
50L + 150H = C,
L + H = 1
If you rearrange the bottom equation to solve for L, you get L = 1 - H, which you can substitute for L in the previous equation:
50(1-H) + 150H = C
which reduces to 50 + (150-50)H = C.
Now you can choose C (say 80 mg/dl) and use this formula to find the amount of 150 mg/dl (high concentration) solution to start with. Then you just fill the container up to the 1 dl mark with the 50 mg/dl (low concentration) solution.
You can set this up using any concentrations you have lots of - even odd ones you concoct in earlier mixing (for example, how much 82 mg/dl solution and how much 119 mg/dl solution do you mix to get 1 dl of 109 mg/dl solution?)
Hope this helps,
Stephen La Rocque.