It most certainly depends on the alloy content most directly. Thats why transformation temps are most accurately spoken of in terms of temp ranges. Some alloying elements can greatly effect the transformation temp with very small variations in volume percent. For example carbon content in carbon steels, or nitrogen in SS's.
You are right about the change of critical transformation temperatures depending on alloying content, great book about this is
"the effect of alloying elements in steel", by Mr Edgar Bain, metallurgical legend, an old and genious book indeed. It is somewhere downloable via
pdf, look for it is great piece of metallurgy history.
But, the differences on PWHT temperature, are mainly because of the "resistence to softening", impaired to the steel mainly by carbide forming
elements such as Cr, Mo and V. If you look to some "tempering curves", or diagrams showing decrese in hardness (or UTS or YS), versus
tempering temperatures (after a full quench of course), u will see Cr-Mo steels retains most of their mechanical properties even after high tempering temperatures, but most of the time there is a "inflexion point" in which softening will start more markedly, so a good source of non-code, non-official data to determine the right PWHT
for any alloyed steel, or hardenable stainless, is to check the tempering diagram for that steel, (which is generally readily available in steel producer's brochures), and determine
the temperature at which this marked decrease starts, then for the pwht , 30ºC or 50ºC more should be enough (but not tresspassing lower critical temperature!!)
By the way, this resistence to softening is also the reason why Cr/Mo steels are used in high temperature parts of boilers, and are more resistent to creep
Regards