The main function of pork in a game sausage is to get the fat content up, as game tends to be very lean. If you don't add enough fat, the sausage becomes dry and start to fall apart. The proper ratio depends on how fatty the pork is. I worked in 2 commercial sausage kitchens where we processed venison. We'd mix about 2 parts venison to 1 part pork. But, the pork we were using was very fatty -- usually 60% or 80% fat. You can buy this sort of trim fairly cheap (a butcher should be able to order it for you, and you'd probably have to take a case -- as I recall, you can get smaller cases of 80% fat pork). Anyways, if the company recommending 3 part pork to 1 antelope is recommending pork that is 30% fat, you could cut that in half if you got some pork that was 60% fat and make your sausage more an antelope sausage.