My estimate, which of course can't be 100% correct but this is what i see.
Dark Souls is HUGE on PC, definitive the bigger platform for Dark Souls.
Dark souls is at 1.5 million in Nord America as for 10 May, right? It sold about 850K on Steam which mean is definitively >1 million on PC worldwide, most likely even over 1.2 million, and i can see about 40% in NA or so, which could mean 500k of those 1.5 million. This leave 1 million shipped on PS4/XBO.
Let's say that, about 80% of those shipments were sold, which seem likely... this is about 800k.
Don't forget that even on console it's digital sales were immense, look at Japan, and North America is most likely even bigger.
It's easy at least >20% digital, i'll go with 25% or more. So, this leave under <600K at retail on PS4 and XBO.
As well, remember we talk about North America, not USA... USA is usually 90% of NA, so i think it's <540K in USA.
In conclusion: i see between 500-550k for Dark Souls III first month at retail on PS4/XBO, more close to 500K than 550K for the reason you saw above.
I made i typo with ReC, i wanna say >250k, not <250k... still, even if will sell amazing well and break the 300K mark, i think The Division will be 350,000 this NPD, so it won't beat it. Imo, of course.
Is impossible to know if Dark Souls's biggest plataform is Playstation or PC, from the 8 Millions units sold of Dark Souls 1, 2, over 3.25 Millions were for the PC.