The dose should depend on how big the horse is and what type of infection you are trying to treat. Usually something like 1.5-4 mg/kg/dose and then (at least in humans) we would follow blood levels since too low of a dose doesn't work and too high of a dose can cause kidney failure.
OMG gentamicin is not something you want to use the "wing it" method to dose! It is weight-based, and the frequency of dosing also depends heavily on how well the kidneys are working. One size does NOT fit all--this is a nasty drug to get wrong.
I am sure it was calculated and re-calculated. I just saw it on the discharge papers after I got home and the vets were gone for the day and I wanted to triple check before giving it to him in the early AM.
He will only be on it 3 more days then switch to oral meds.