went to the range today with the .270 and 30-30 and it was a 15yd range. the guy said if i aimed at the bullseye and i hit an inch low that at 200yds that it would be perfect. is this true?
That sounds pretty close, at least for the .270. For a 100 yard shot, the bullet crosses the sight line the first time at around 25 yards- usually if you are on at 25 you will be very close at 100. Zeroing at 50 yards will put you 3-4" high at 100, and right about on at 200, same should work for 15 yards with it being a little low at 200.
The 30-30 will be much further off at 200- after 150 yards, the bullet drops very fast.
You need to be dead nuts on at a shorter range though, a 1/2 inch off at 15 yards will put you at least 7" off at 200. You will also need to verify your zero at the range you will shoot it at if you have any hope of cleanly harvesting game with it.