Dimensional analysis is hard to show on here without fancy fonts, but I think the lbs/Newtons here is either backwards or I'm reading it wrong.
1 lb-force = 4.45 Newtons.
So divide your motor's average thrust by 4.45 to get the thrust in lbs. Then divide this by 5 to get the MAXIMUM recommended liftoff weight of your rocket. (Or, divide by your rocket's weight in lbs to get the ratio)
So an N5800 exerts about 1300 lbs-force, and therefore could lift 1300/5= 260 lbs. (approximately 1 snowmobile).
OR, if you use your N5800 to launch an Estes Alpha, which weighs around 0.05 lbs, your T/W ratio is about 26,000:1, which is more than adequate, even on a short launch rod. Of course T:W may not be the only thing to think about here. :wink:
I think this is right.....
So if you were an RSO in a hurry, aside from slowing down, you could divide the motor's average thrust by 25 to get in the ballpark of a maximum safe liftoff weight and to decide if you needed to look more closely at the rocket/motor specs. But really, slow down.