When I teach dilutions, I suggest to students that they draw a picture of what is going on. With respect to your example, I suggest starting with the idea that the mass of material in the pipet tip (1) is equal to the mass of material in the well (2). Putting this idea into a formula: Mass1 = Mass2. The mass of TNFα is equal to the concentration of a solution times the volume of the same solution. Try writing an equation that expresses this idea. Making a picture minimizes plugging incorrect numerical values into a formula.