You realize they package it for smaller animals too? Not that I am saying you should take a medicine that was packaged for livestock, but if you do it is easier to meter out the correct dose with something like the goat formula rather than horse.
Five 50-pound increments are about an inch on the plunger.
There is no problem squeezing out 250 pounds worth.
There would be no problem squeezing out 50 pounds worth and giving it to your dog instead of paying the vet $10 per month for it.