85, all I've ever used to duplex loads is SR-4759 and with that the most I've ever used is about 7 grs. and that with a 110 gr. charge of Ffg in a 500 BPE. The regular charge for that cartridge is 136 gr. in the old balloon head cases.

I have read about your '3-1' ratio and also read never to exceed 10%. I've always used the 10% rule and IIRC, have never exceeded 7%-8%. More often than not 5% has been adequate and, I've never experienced any difficulties. I'm positive either method works and probably works well. As you state, I agree the age and strength of the action matters but I don't agree that doubles are necessarily weaker. Really old ones, yes but, since its introduction doubles have been chambered in 9.3 X 74R, 375 H&H and any number of much higher pressure cartridges such as the 8 X 57, 30-06, ad infinitum.