Do we know that PS4 is 2:1 in the US? As far as I understand it's just hearsay on Gaf. If true and PS4 is selling at a rate of 3:1 on the Xbox, the whole situation is a disaster for Microsoft - it's not like they're making any more profit on each machine than Sony either.
To be honest with you, it's a bit of a sad state of affairs; if Sony completely monopolise then there's never going to be much of a need for them to ever excel with hardware again.
Both companies bet on camera tech this time around and it really hasn't worked out... Bring on E3, I want to see Microsoft throw everything at it (minus American Football, TV, etc.)
Here's how you would do the rough calculations. These are estimates of course.
A good assumption I think so far for X1 is it'll do about 60% as many sales outside US as in. I think that number came from when they announced 3million WW sales, you could compare to NPD at that point.
So you could guesstimate it sold .6X115 outside USA in April, which gives ~70k, so USA +world~185k. Working with such gross estimations we could be tempted to round it off to 200k.
For PS4, it sells much better in Europe, probably ~1:1 with USA, or perhaps even better. So right away you're at 200k (NPD) +200k (EU estimate)=400k. Plus throw some in Japan, round up a lot (probably warranted with strong international sales), maybe you're at 500k.
That's basically all it is, but it's probably a decent guesstimate.
I know for 360, MS worldwide shipments tended to be about 2X USA NPD sales throughout. So that's another guide. I mean it held right to the end, 360 was at 84m shipped and I think 40 or 41m sold in NPD lately. XOne probably isn't far off that pattern, last ship total at March was 5.1m, and walla, NPD LTD was ~half that.
Ship totals probably have a lot of cruft in them though, so I wouldn't necessarily assume Xone is currently doubling NPD sales worldwide. Even if it was that's only 230k which isn't grossly off from our estimates.
Just thinking, it probably works for Sony ship too. PS3 did ~25m LTD in NPD IIRC, and is probably around the same 80-85m WW ship as 360. 3X25m gets you to 75m or close enough (PS3 NPD LTD is probably higher than 25m too, dont have those numbers offhand).
So by that basis Sony might have sold 600k WW on 200k NPD, but using that higher Ship:NPD method would put X1 at 230k using like for like estimations. I think that's reasonable. Either way it looks like 2.5/3-1.