If you aren't confident estimating range, you can sight in your rifle to maximize "point blank range" for the target you're shooting at. As an example, a .223 sighted in at 100 yd will be WAY low at 250 yards. However, if it is sighted in at 200 yd, it will be only 1.5" high at 100 yd (IIRC), right on at 200, and only slightly low at 250. This means that you can hold dead on a 3" target all the way out to 250 yards or whatever. (I don't have exact figures at the moment, but these numbers are close.) There are several ballistics programs that allow you to calculate "maximum point blank range" or MPBR for a particular target zone; Oehler Ballistic Explorer is a good one, and the freeware demo will give you a feel for the concept.
Also, if you choose a flatter-shooting caliber like .270, you won't have to worry about knowing the range as precisely.