Your calculation looks right. Though you don't really need to go to the hassle to measure out the paste with scales if you just use the bodyweight guides on the syringe itself.
For a 0.2 mg/kg standard dose I calculate that your tube would be for 1227 lb of total bodyweight, and taking into consideration rounding errors I'm guessing it is marked as for a total of 1250 lb bodyweight. Am I right? If it is marked as for a total of 1250 lb bodyweight then just dose for your own bodyweight using the guides on the syringe and pretend you are a horse! (the 0.2 mg/kg bodyweight dose is generally standard for both humans and horses)