A pound is a pound no matter what is being weighed. However, the horse dose is on the lowest side of the range at .2mg/kg. For non cancers the range is .2-.6mg/kg so double it up and then some which is probably a 500lb horse dose.
But might I suggest, go out tomorrow and get your dog the pure stuff, it’s the injectable ivermectin. Get a 23ga needle and draw up about 3.5ml. Put it on a spoon with some honey or peanut butter I guess, and repeat for 5 or so days