Not sure where you hunt,but, .243 is plenty where I am. Mature bucks average around 150 pounds, so a .243 is more than enough. I've used both .30-06 and .243 on this ranch, and the .243 kills them just as dead, just as fast. I've never needed two shots with either. I use 100 gr. Core-Lokt's in the .243 and 150 gr. Power Points in the .30-06, and get complete pass-throughs with both. Maybe if you're hunting where the bucksgrow to 400 lbs., you might need a .270, but, on average, bucks weigh 250 lbs or less and a .243 is plenty for humane kills. And, it seems to me that arguing for a minimum of .270 so you don't risk wounding a deer with a puny .243 is just an excuse to make up for poor marksmanship.
Just wanted to argue about this statement a bit:
You had only better take very high percentage shots with it.
Seems to me that's all anyone should take. If you're taking marginal shots, then you risk losing the deer regardless of what caliber you're using. Just doesn't compute for me if somebody thinks, "Hey, I've got a new whiz bang super-mag with the equivalent knockdown power of a howitzer, I'm gonna make that 'Texas Heart Shot'." Everyone who goes into the field should know the limits of their ability, and the limits of the caliber they are using, and limit their shots accordingly. If everyone didthat, the deer, the hunters, and the sport would all be well served.
As far as determining effective range, what I've heard is that 1000 ft lbs is a good rule of thumb. So, the effective range for a .243 would be a little past 300 yards.