I don't much use 243 on deer but have shot three in the past. My ex killed several with her 6mm Rem which I only loaded 100gr bullet's for. Two of mine were 100 gr and one was 75gr. A 75gr is definitely a varmint bullet and I wouldn't recommend it. If I were to go back to the 243 for deer, for me it would need to be 87gr and above. The 87gr and above bullet's maker's clain re made for deer. Sighting in is very important. I use the max point blank method and seldom shoot to 300yds, but I did one time! Not with a 243. To use MPBR, you figure the size of target you want to hit. My 243's are set with a 6" targrt. What that means is the bullet will never rise over 3" or fall 3" all the way to the MPBR. To get there sighting in at 100yds works well but, you need to know where the bullet path is to know how high to zero at 100yds, It will not be right on.
I have a MPBR I figured up for my 22 mg right here. 40gr bullet with BC of .095 got the BC off the internet. Muzzle velocity is 2200fps. The size of the target is 4" MPBR is 147yds. At 100yds the bullet will be 1.7" high. at 147yds the bullet will be right on and at 150yds, (as far as I got the data for) the bullet will be 2.1 inch low. What that means, is if you sight in at MPBR with a 4" target I'm good to hld right on out to 150yds and the bullet will never rise above 2" or fall below 2" from the muzzle all the wat to 150yds. Well 2.1 low at 150.
Take most high power bolt action CF rifles and sight in on an 8" target and most will either still be on the target at 300yds or just a bit low. Think of shooting through a steel pipe that inside measurement is 8". You start with the rifle zeroed to some point where the bullet never rise's over 4" and never falls 4" with out touching the top or bottom of the pipe! All my rifle's are sighted in this way and out to 300 yds there is little if any hold over. At 300 yds the most I've have to hold with any of them is about 7", pretty easy thing to do. The target area on a deer is what, 12"? shoot at it with a MPBR at 8" and your good to go. I do adjust the size of the target to get the most out of the rifle. You notice with the 22mag I used a 4" target, I use a 6" target with my 243's and the rest either 6" to 8" depending on the cartridge.
In my youth writer's always said to zero your rifle to be 3-3 1/2" high at 100yds and you'd be good to go. That was the days before it was called MPBR but that is what it turned out to be. To figure it you need some kind of computer program, couldn't tell you what. My chronograph figures it out for me but, after getting the data you need to shoot against it a various ranges that shot on the read out. I also confine my shot's on game animals to about 350yds, only did that once farther, 330yds, and did it just to say I did! If Iwanted to go further than what I have it's a simple matter to feed the data bck into my chronograph and get to 1000yds and not sure i=of the increment's but I;ve used down to 5yr increment's to get it right for 500yds for my 6.5x06 with a 140gr bullet, scary accurate!
The 243 will certainly work on deer, anyone that say's different is simply fooling themself. I don't use it much because I have what I believe are better cartridge's.