When building a model you have nearly always have to make major assumptions that drive the output. Obviously you make your best directional estimate based upon available data. However, as you are finalizing the model the right step is to assess the sensitivities of your output to your assumptions.
One good way to show those model sensitivities is in a tornado chart.
First, select two series of data that show your max and min range for each sensitivity variable, and put them in horizontal bar chart, like below.
Remove all the normal Excel chart junk to get a cleaner view of the data.
Next, Format the x-axis and click on the Scale tab - set the "Category axis crosses at" variable to 20 (in this case - whever your midpoint is).
You should have what looks like the above. Now, right click on the right bars and format them to be the same color. Then, click on the options tab and change "overlap" to be 100. It should look like the below.
Here is an excel file with the below tornado chart example.