The application is not really set up to do this as everything is date dependent. However there are a couple of ways to sort-of cheat.
Set up an Unscored measure as an ‘Annual’ measure. Then set the actual value of the measure to your Constant (e.g. π). You can then reference the constant value for 12 months. The chances are you will need a new value in 12 months so this is the best alternative.
Set up a Goal/Red Flag measure and set the ‘Goal’ to your Constant. Set the Red Flag to Zero. You can then use the Goal value in calculations. the Goal value will persist for as long as the measure exists. This example is a bit clunky, as you have to remember to use the ‘Goal’ on the calculation.