Fortune Magazine once did a survey of the Fortune 500 companies, examining the essential planning spreadsheets they used. In 80% of the companies they found major flaws leading to significantly incorrect predictions.
In the light of this and since I haven't noticed the capitalist system coming to an end just yet I've significantly revised my approach to spreadsheet modelling.
Here's what I do: Make the spreadsheet look very complicated. You'll need an Input Parameters page, several Calculation pages and a Summary and Results page. A Graphical Output page filled with incomprehensible bar charts and pie charts adds verisimilitude to the whole thing.
The calculation page should be linked to the input parameters, but the calcs can be anything, just so long as the numbers change when you input new parameters. Then delink the Summary and Results page from the rest of the spreadsheet. Link all the answers to a concealed factor (I call mine the "Rainfall in Timbuktou Factor").
If the boss wants to change something just change the relevant input values and adjust annual rainfall in Timbuktou until the answer looks like what the boss wants.
Provided one's boss is a bigger computer illiterate than one's self it's easy and you get to go home on time.
Go on. I dare you!