You've determined the range you expect to engage targets.

Can the scope compensate for the maximum range ?

This means that you need to determine if there is enough holdover on the reticle, or if the turrets have sufficient adjustment range. This means you need to do some homework. Quite a bit of homework, actually.

At this point you need to become familiar with ballistics, because you need to be able to understand how to read a bullet drop table for the load you expect to be shooting at targets with. The bullet drop chart will tell you what drop to expect at the maximum range you shoot at. The choosing of a load is a whole process in itself. If you are unsure, pick a mid-range load with the most readily available rifle bullets for that caliber that have a BC of at least 0.4 . That is a good point to start, and you can refine your bullet choice over time with experience.

Then determine if you have sufficient holdover with the reticle, or if the turret elevation can adjust that far. For example, you need 25 MOA holdover or elevation adjustment for 155 grain bullets in your .300 Win Mag to reach 1000 yards. A scope has an elevation adjustment range, only half of the total of which is available on either side of the zero. To elevate 25 MOA, you need a scope with at least 50 MOA total elevation adjustment. If your scope elvation adjustment is lacking, and you can't afford a scope with greater adjustment range, you can find a rail or scope rings that have the required MOA adjustment built in , and this is cumulative with your turret elevation capability. For example, a scope with a total of 50 MOA elevation adjustment, and a 20 MOA rail, gives (50/2) + 20 = 45 MOA total elevation capability. MOA rails can be found for all popular rifle actions, and range from 10 to 40 MOA.

Now decide what magnification you need. Keep in mind need and want are two different things. In practice, you don't need a lot. Long range ( 1000 yard ) competition shooters get by very well on a 14x magnification.

You will find that beyond 14x magnification, the quality of the glass suddenly becomes very important, so choose a higher quality sight picture with a smaller magnification over a lesser quality sight picture with a higher magnification, and you'll come out ahead every time.