I like using the Math Module cards for situations like this. They perform the calculations required to get you the outputs you need. You simply install them in to the next slot adjacent to the module you need to alter. For example, if you had a 16 point output module, but only needed 8 outputs, you would need a 'divide by 2 module'. In your case, you would need to purchase and install a 'minus one' module. The neat thing about these cards is that you can calibrate them with a calculator.