I can't see well enough to shoot at 300 yrds anymore.
I'd recommend seeing at what distance you can keep an acceptable group, then set your sight-in to include that furthest distance in a Point Blank Range calculation. BUT, then go the range and make positive sure what your rifle is doing.
My best deer ever fell to a .243 100 gr handload with a pretty vanilla bullet by today's standards. It was a 100 gr bullet with a "Winchester" name I bought from Midway. It was a very modest charge of IMR4064 and modest FPS but bullet placement was the key. In '95 when I shot the brute my ability to shoot distance was limited to about 250 yards ... fortunately the deer trotted broadside only 80 yrds away.
Rather than get too technical go to the range and have some fun.
If you don't have access to shooting more than 100 yrds then try to extrapolate the info by using some of the good ballistics charts available free online.
Good luck. I love the .243. Chances are that you will get most shot opportunities under 175 yards easy, unless you are shooting some bean fields or open western country.