Re: 7mm MAG ?
Sharpshooter, the only way to know if the load will work at longer ranges is to shoot at those ranges. Stringing, and a host of other issues may make a good close range load unacceptable. Besides, you are going to need the practise anyways.
The 7rem will work at your ranges. The 139gr SST is a great bullet but also consider the 154 and 162 SST. Reason is wind drift and retained energy.
The heavier bullets have higher BC which will help reduce wind drift and make long shots easier. Also, a heavier bullet will always have more momentum, and at long range you need all the oomph you can get.
A 162gr SST at 3000fps will make a much better load, for me, then the 139gr SST.
I practise on gongs, clays at 700yds out to 1100yds. We use smaller targets so that it improves our shooting skill and allows for some error when we engage game. It is a lot of fun and you learn quickly about load development and the wind.
Make a drop table calibrated for your range finder/scope, practise lots, and good luck this fall.