The Calculator is simply a handy tool that we have built-in to Dispatch that you can use instead of relying on the calculator program that is usually installed with Windows. The Calculator is shown below.

You can insert the results of a calculation into the control that was focused prior to opening the calculator by performing a calculation and clicking the Insert button. For example, if you are entering a Net weight value on the Ticket Editor and you know the Gross and Tare weights you could do the following:
Click on the Net column.
Click Tools > Calculator to open the Calculator dialog.
Calculate the Net weight.
Click Insert to put the result of the calculation in the Net column.
Click Close to close the Calculator dialog.