Have you tried running the Barrowman equations to check your "new" design?
I am assuming that as a fresh BAR, you probably have not yet spent $100 on a copy of rocsim. The Barrowman method is free (posted several places on the internet) and fairly simple to run.
This will present a problem for tube-fin designs, however; the Barrowman method includes instructions only for "conventional" flat-finned rockets. For tube-fins, I would suggest you use the tube diameter for fin span, use the tube length for fin chord, and if the tube-fin design has six stabilizing tubes then use a fin count of six. This is a conservative modeling approach (with a stability estimation method that is already conservative) and if the results say your rocket is stable it should be just fine.
Barrowman will give you an estimate of the location of the center of pressure (c.p
.) and you will need to measure your own center of gravity (c.g
.). Check the c.g. location by prepping the rocket for flight, including new motor, wadding, parachute....whatever is on the rocket 0.01 seconds after iginition needs to be loaded for your balance test. Hold the rocket horizontally, balance it on your finger until it sits still, measure the location of that balance point and you have your c.g. location.
Compare the c.g. to the estimated c.p. and make sure the c.g. is ahead by at least 1.5 to 2 body diameters. If you need to add nose ballast, soft modeling clay works well for model rockets. Drill a small hole in the base of the nose cone (the internal
part of the NC, the part that is hidden inside the body tube and is facing the parachute) and pack in some clay toward the tip of the nose.
These "rocketry 101" skills are something you should be able to do if you want to modify kits from manufacturers, or create your own designs.
I hope some part of that helps-