Excel can be a chemical engineer's best friend, but it can also be madding when it doesn't work. If you're having trouble with circular calculations, check out this tutorial. It demonstrates how to use circular calculations to perform a two-way case study using the Colebrook equation.
This Excel tutorial for chemical engineers is part of the Learn ChemE series on YouTube. You can find hundreds of tutorials on a wide variety of subjects. Check out the the range of help available to chemical engineers and chemical engineering students here.