Are you sure every mark is 50lbs? What does it say on the box? All the horse ivermectin syringes I've seen have marks every 250lbs. So, like only 7 marks on the syringe.
If you accidently take 750lbs worth, you probably won't die, unless you're allergic. But you will feel groggy and shouldn't drive if you take a lot.
I weigh 200lbs and took 250lbs worth once a week, for 3 weeks.
The above link recommends 68-91 micrograms per pound of your bodyweight. It depends on what disease you have. I weigh 200lbs, so 61mcg * 200 = 13,600mcg, which is a 13.6 milligram dose.