Digital Microfluidic Biochips (DMFBs) can execute biochemical experiments very efficiently, and thus they are drawing attention recently. In biochemical experiments on a DMFB, “sample preparation” is an important task to generate a sample droplet with the desired concentration value. We merge/split droplets in a DMFB to perform sample preparation. When we split a droplet into two droplets, the split cannot be done evenly in some cases. By some unbalanced splits, the generated concentration value may have unacceptable errors. This paper shows that we can decrease the impact of errors caused by unbalanced splits if we duplicate some mixing nodes in a given dilution graph for most cases. We then propose an efficient method to transform a dilution graph in order to decrease the impact of errors caused by unbalanced splits. We also present a preliminary experimental result to show the potential of our method.