Uhh, I don't know - ask RockSim.
Anyway, like I said, if you use comparable proportions and straight scaled-ups of the fins in your build, then the CP location should be in about the same proportional location as it was in the original Astron Sprite. It will be the CG that will vary due to the difference in parts densities, etc. In order to adjust the center of gravity to a suitable location, you must always find the location of the center of pressure first. You can adjust the CP's location by changing the length of the airframe or the size or sweep of the fins, but most people find it easier to adjust the location of the center of gravity instead. As a result, the CP's location is usually thought of as being stationary (fixed in place by the overall design), while the CG's location is regarded as movable (changed by shifting the distribution of mass).
Dave Rose posted an
article on EMRR about his scratch-built Superscale Sprite, which was also based on LOC 4" tubing. You can find his RockSim file
here.
MK