Thanks for that, I stand corrected on the mixing/reversing terms of impulse and avg. thrust.
For safest: check the thrust curve, etc especially in marginal situations.
Here's excerpt from NAR high power cert test pool which kind of relates to rule of thumb
"A10) What is the maximum allowable weight for a high power rocket permitted per NFPA
1127?
A) 100 pounds
B) 400 pounds
C) 3069 pounds
D) There is no limit provided the rocket weighs less than 1/3 of the average certified thrust of the
motors intended to be ignited at launch.
-----------------
The answer is "D". Refer to NFPA 1127, 2008 edition paragraph 4.9.1."
As a mechanical Engineer I use a number of NFPA and NFPA derived standards, they always have an excellent margin of safety.
Thanks for that report, it's now printed and cached in my rocketry bible binder