I usually just build then weigh the final product. Weighing each individual screw, rivet, drop of epoxy...that's crazy. Build the thing, weigh it, measure the CG and go forth. Only issue is when trying to add nose weight to the nose cone. But I actually get the mass and CG of the nosecone and the rocket minus nosecone separately. That way I can add mass to the nose cone to get more accurate sims. I mostly worry about stability and speed off the rail because 10,000 feet is way above anything I am flying and on some rockets I miss the altitude be a few hundred, some hits within a few feet...on the same rocket.
Measurements do no have to be metric.
The AT J350W is not a single use motor.
Do you know the zero mass tailcone trick for short stubbies?
You do not want to put nose cone weight in the Cowabunga to bring it up to 1.5 cal stability and especially 2 cal. You would be looking at around 12 to 20 ounces in the tip. Mine has some weight in the nose via an electronics sled; and it did have 2 oz of fishing weights on the sled. But I got rid of all that, streamlined my sled and poured 5 oz of sand/epoxy around the bulkhead in have in the shoulder. It has about .9 cal stability empty, .4 cal with an "I" in it. Flies straight up, even in moderate winds. It should be able to fly a 38mm baby J like that. But I don't know how much weight you added to the tail end portion via your fiberglass and what not.
The standard nylon recovery harness that comes with that kit will suffice. Did you order the chute and chute protector with it? If so then you have everything you need and it is probably a thin mil chute that can pack pretty small. You are still gonna be tight on space cramming a J in there. It can be done of course, just practice the technique. Or you can order a payload kit from Madcow and stretch it a foot or two for the level 2 flight, just to be sure you have plenty of room.
Depending on how much weight you added, a 30" chute may bring it in pretty fast.
I agree there is a lot to learn and I really don't see why you would want to hurry along the level 2 just for the sake of doing it, especially if you rarely fly. Get your level 1. And between launches build you a level 2 bird. It will keep you engaged and you will have another bird to fly. Then after level 2, stick a J in your Cowabunga and let er rip. But it is you doing it so you do it how you want to.
But I am not one to talk about restraint. I rarely fly and I just recently got my level 2, but I enjoyed my down time between launches by building the bird for my next cert flight. The second (or was it third) motor I assembled was for my level 1 flight. My level 1 flight had electronic deployment and my level 2, which was barely less than a year and maybe 3 flights later, had dual altimeters and GPS tracking. Every flight above an F has had electronics for deployment. And soon even my Big Daddy will have an altimeter.
The main thing that will get you is the recovery. Learning to pack you chute and harnesses, and that only comes with practice. You can fold and pack on the ground until you are blue in the face but when pieces get to spinning and flopping around on descent, things get tangled, broke, dented, things just happen.
Get it flight ready minus motor. Weigh it and balance it to get your CG. Post those numbers here. And also if your fins are rounded, airfoil or square. That will go along way to getting help from the good folks here. Get a good handle on the OpenRocket program as well and that will help tremendously.
Mikey D