And it is always a good idea to have some understanding of the basic calculations before you blindly use someone else's simulation software
You can find the basic Barrowman calculations for center-of-pressure in several places online (for free):
https://my.execpc.com/~culp/rockets/Barrowman.html
If you have any problems I will be happy to answer qstns, just PM me and I will step you through
And I add my 2cents to recommending Stine's
Handbook of Model Rocketry (any edition, although the latest is the best), this is a great all-around source for how to perform many of these calcs.
I should have been more specific in my criticism. Sorry.
To be objective, Stine makes four primary mistakes:
First, he uses an average mass approximation, which works over very small intervals, but normally not over the entire operating interval of a motor. He is saved, in his LPR computations, by the very small mass fraction of propellant. That is, his methods were poor (and the methods are the object here), but they made no significant difference in his results in this case. (A simple remark that this is appropriate because of the low mass fraction would be an adequate correction, here!)
Secondly, he forgot to include the effect of gravity in the boost phase. This makes a significant error in his calculations, and its also a flaw in his method. (This is easily corrected.)
Thirdly, he compares the results of his computations with those of his RASP simulation program but the thrust curve in that simulation program has a different impulse from the one he was using in his computations. He blames the discrepancy on the thrust curve shape. In fact, the thrust curve shape makes extremely little difference if you do things right. This is a qualitatively wrong inference. (Simply changing the working impulses would correct this problem.)
Finally, his simulation program isnt very good (because of his coarse intervals for small motors, and his use of velocity at least interval to compute drag) so it makes a poor reference. (Using a shorter interval for small motors would help here.)
The second and third problems tend to cancel each other to a certain extent but two errors dont make a correct result.
This stuff is easy to correct. Given that the book (which is very good!) is such an important document, it really should be corrected. That's all I'm sayin'...
Best Regards (Not comin' atcha, and not thinkin' you were comin' at me) C.