Regardless of the halide ratio, the absolute Ag+ in solution is the item of concern. With that in mind, pAg or vAg then becomes the measure of choice. With mixed halides or between Cl/Br/I and mixtures thereof the pX can be just about anything, but equimolar Br and I for example alone will give nearly the same pX or vX, but the vAg or pAg may be different by a huge amount. Therefor it is easier to measure Ag+ with precision by use of the Silver ion in solution regardless of halide type or mix thereof. This then determines excess halide, of whatever type, in the coating at the start of the process, and also helps with keeping.
Of course, I like to prewet which normalizes vAg to be about that supplied by city water which actually seems to do nothing with D-76 or HC-110. Now, I have to try Rodinal.