I would argue that there is no distance where it makes sense to move up to .30 cal since it is honestly one of the worst long range calibers out there regardless of which cartridge you're talking about. For shooting paper, the .243 will outshoot the .300 Win Mag with less than half the recoil out to well past 1,000yds. How well your .22-250 will do at longer range is largely going to be determined by the barrel rate of twist which will dictate the heaviest, highest BC bullet you can shoot from it. If it's the standard 1:12 or 1:14, wind drift will limit your effectiveness to around 400yds most likely. If you have one of the much less common 1:9 rifles, you can step up to heavier bullets and comfortably work out to 600+ yds. If you want to go farther than your .22-250 is capable, a .243 with a 1:8 twist barrel or a .260 would be the logical step up. Either will shoot circles around a .308.