powdr, I'm sorry I have no (good) advice to give you. There is no way to calculate the optimum powder, and none of the cartridges which I load are similar to your .338-08. Were I in your shoes, I'd do pretty much as you propose. From .338 Federal load book data, pick the powder which looks best and work from that--and consider also trying a bit heavier charge of the next "slowest" powder shown in the .338 Federal data. For such a small difference in case capacity, I can't see being more complicated than that. Barsness' 4:1 rule is as good as any; use it and your chronograph.
You pointed out Ackley reckoned his blown out cases required 4 to 7 % more powder. My hunch is the upper end would be for single base stick powders, and the lower end for double base ball powders. Hodgdon has a large amount of data showing pressure for both maximum and starting loads. The trend I see is that the double base powders have a faster pressure rise with increasing charge. Like all else in internal ballistics, that's not a solid "rule;" I'm certain I can find exceptions with a few minutes of looking.
You also mentioned the numbers I tossed out "gave some validity to what I've been doing all of these years". The fact you have your fingers and your eyesight after years of wildcatting is more meaningful than my numbers --or maybe it's a testament to the quality of your guns. The numbers I gave are only coarse approximations.
