By my crossbred calculation, that would be 0.2ml per gallon. My DS calf rennet, according to the label, recommends a max. of 1.25ml per 10l milk, that is 0.5ml for 4l (=approx 1 gallon). Why not a small make, say 4l(1 gallon) and see what happens - if its really slow to coagulate then you know you need more.
Margaret