How to calculate a rocket's propulsion (aka What am I doing wrong?)

[stix]

These are for a 12" dola so i dont want them too nippy or it`ll rise too fast and be too high for anything except salutes

 

Ah, I see - didn't know what a dola was until I googled it. Very impressive. Sounds and looks like a lot of fun Col.

 

Cheers.