My rule is simple: a separation of give or take 50gr in bullet weight for centrefires. So 50gr, 100gr and 150gr, and that equates to .223, 243 and .308. I’ve had those three cartridges for 30 years give or take and there’s nothing in the world of medium game I’ve not been able to take on.
For those periods of time that I’ve chased somewhat bigger animals than large deer, I’ve simply stepped it up another 50 or 100gr, to 200gr and 250gr, and taken up medium bore rifles of which the .35 Whelan in Aus was the most memorable.