I think we all try to determine the range according to several things.
The accuracy which your gun is capable of,if you can get minute of angle groups with your hunting load then its probably more accurate than most people can shoot under field conditions.
The way you are sighted in. I personally believe in the 3 over to 3 under method so I sight in for 3 over at 100yds. With my hunting load that puts me 3 under at 200 yds.
[140gr BH a blue 50/40 sabot and a Shock Wave or Lehigh bullet 200gr]
I can shoot 100 yds off hand with confidence but will find a rest to go over that.
Know assuming that you are a good shot and sight in 3 inches high at 100 with your load you will be about dead on at 140 and 3 inches low at 160. Now guns vary so do people you need to check this out at the range, Hornady makes more than one 250gr I assume you are using the XTP if you are using the SST or the SW it will be shooting flatter.
AS for the number of pellets very few will shoot 3 accurately in fact most of us use loose because you can tune your accuracy better with the ability to change the powder load to what the gun likes. If you are getting close to inch groups at 100 yds then don't worry about it and don't change any thing.
ALSO THERE is the fact that you should not exceed there max amount in you operaters manual for that gun. The only Spanish barrel I would even consider using over 110 gr in is the Bergara.