Spools

From CD Spool Rocket Resource OddrocWiki

Spools function mostly because of their high base drag. If you want to simulate a spool simply setup a pair of "flat" transitions (front and rear diameter within 0.0001 inches) and a motor tube. Then add the same base drag correction for short stubby rockets - namely a "massless" transition of pi * base diameter in length. This pseudo transition is only a fiction used to trick RockSIM into appling the right drag force. The transition starts out just at the base of the rocket as a tiny "point" and grows until it is the same diameter as the base of the spool.

Using this model should get you within the ballpark but you should still call a heads up flight each time you fly a new motor/spool combination.