This is how I test the charges.
First I don't use my altimeter - I use a wire with a battery and a switch.
To hold the rocket I have a bike stand. It has an arm that has a clamp that fits nicely around rocket body tubes. I put a little rubber around the tube to give it better grip. Also, I hang about 50lbs off the bottom where the supports for the tripod start, and but a bag of sand on each leg. If I'm testing either the fincan or nose cone the procedure is the same. Load the ejection charge and put on the fincan/nosecone. I put some dog barf in there to lessen the flame damage. The I stand about 3 feet from the nose cone with a helper and big king sized sheet. We both stand so that the rocket is in the middle of the sheet between us. I stand on one edge while holding the other edge of the sheet high in my hand. Works best if the sheet is slack to absorb the impact.
Then I give a brief count and hit the button. Nosecone/fincan shoots into the sheet and gently decelerates. Kind of like throwing and egg into a shirt on a laundry line (where I got the idea).
My 2 cents.