Initialising datablock values set by script

I know that in a datablock's constructor, there's an opportunity to set default values on all its members. So if something's not set by script, there's a sensible value in there instead of junk. However, where should I initialise values that have already been set by script?

The specific example is that I'm working on projectiles bouncing. I only want them to bounce if they hit a surface under a certain angle. The angle is a datablock property. To make it easy to use, I just make the user set an angle in degrees - but in-engine, I need to have the cosine of the angle, rather than the angle itself. Where is the best spot to perform a cosine on the angle, after it has been passed in script?