It's all about average thrust and thrust curves, Bow.
If you go to
Thrustcurve and do a search for G80, you can go right to the NAR Data Sheet for the G80. You'll see, when you scroll down to the thrust curve, that the G80, almost instantly, jumps right to 100 n-s, or about 22.5 pounds of thrust. This point is usually called peak thrust. It gives the rocket enough kick to get it moving. That why you'll notice that a rocket on a G80 and a rocket on an AT G35 will take off at different speeds, the G35 a lot slower than the G80. The Ellis G35 only peaks,
according to the TRA, at about 12.5 pounds of thrust. A lot, lot less than the AT G80. The reason it has 124 n-s as compared to the G80 at 120 n-s is because it burns slightly longer.
Really, all you are looking for when you select a motor is it's max liftoff weight, to see if it can get it off the pad in a timely and stable fashion. An easy way to find the max liftoff weight (MLW) is to find the peak thrust of the motor using a thrust curve, estimate it out (for the Ellis G35 it's 56n-s) and divide that number by 4.45. 4.45 is the number of pounds in a Newton-Second, correct me if I'm wrong. When you divide that number by 4.45, you'll get 12.58 pounds. This again is it's peak thrust. Then divide this number by 5. You'll get 2.51 pounds. This is the MLW that many RSO's will allow for a safe flight. Some will allow a 4:1 or a 3:1 ratio for some of the bigger motors, but a 5:1 thrust-to-weight ratio is the safest.
Jason