If you are going to get your true south by finding 97 on your pole and marking it, then finding 95 on your pole and marking it, then turning your dish to be right in the middle of them, that should work pretty well.
I can say that when I first got a big dish, not correcting for that small difference between my geographic longitude and the longitude of my southernmost satellite, caused me to miss about half of the satellites I was trying for.
The only thing I can say about zenith is that, on some big dishes, it's hard to tell exactly where that is. On my Birdview, for instance, I have to use a big framing square, held against the back of the dish and the rectangular steel of the mount.. On my first big dish, there was a place I could glue a paper protractor, with a piece of semicircular plexiglass over it. That assisted me in a lot of ways. Nowhere I can put one of those on my BV.
I think it pays to get zenith as precisely as you can, using measuring tapes, squares or whatever other tools you need. It will pay dividends in hitting satellites to your far east and west.
