There are a lot of variables. The total wight of the rifle. Barrel length. Bullet weight. Stock design. Whether it has a brake, Where you position the scope on the rifle. But the most important thing is how you hold and control the rifle with respect to your body. For me 3 is marginal shooting a WIn 70 sporter (about 10 lb w/scope no brake) offhand wearing safety glasses. 3.5 to 4" is comfortable. I'm 64 and not very strong.
Assume your 300 Win Mag weighs 10 lbs total with the scope, has a 26" barrel, and you fire a 190 gn bullet at 3000 fps with no brake. 0.0025 seconds after the primer fires the gun >will< be recoiling rearward at about 13 feet per second, If you're shoulder, hands, and arms don't slow it down the scope will hit your eyebrow about .03 seconds later. To stop the rifle your body has to convert most of 25 ft-lbs of kinetic energy into heat in your body. Some will be converted to heat in the recoil pad. Where else can the energy go to stop the rifle? Yes, you can do that and not let the scope hit you, but you can't be careless about it. You have to actively control the rifle and do it without flinching. That takes some practice. Letting your head (with eyebrow) move some along with the rifle and your shoulder increases the distance the rifle can move before the scope could hit you.
If you're woried, videotape yourself shooting the rifle from the side with the scope mounted well forward on the rifle. Put white marks with tape on the side of your rifle at 1" intervals so you can see just how far the rifle and your head moves in each video frame. If the gap closes to an inch more than than the eye relief of your scope then you should be concerned.
The remedies are
1. work on your holding technique.
2. add a muzzle brake
3. get a longer eye relief scope.
NTSC (US standard) videos are .0333 seconds per frame.
