All of the Australian Defense Industies powders (makers of Retumbo) are optimized for temperature stability. few other powders can compare. This is why companies such as Hornady, switched from ball powders to the ADI line of powders where practical.
In the large magnums such as yours, you will find that the heat generated from successive shots is far greater than the heat generated by the outside environment.
By simply leaving a forth shot chambered for a period of 30 seconds (from an otherwise low deviation load), also observing that the barrel at the chamber is indeed quite hot, you will be able to notice signs of heat related pressure problems over the chronograph. A rise of 70-100fps will indicate problems relative to chamber temperatures.
Thanks, Nathan. I'm aware of the issues you pointed out.
What I'm looking for, if it exists, is the kind of 'ballistic stability', for lack of a better term (probably is a better term) that Carlock has found with the 338 EDGE and his use of a certain amount of H1000 that gives very similar fps in a fairly wide range of typically hunting temperatures, say 20F to 70F.
I'm hoping to learn if others have found a such a sweet spot for this chambering in this regard.