PS4 calender year sales vs calender year shipments

[zorg1000]

Teeqoz said:
I thought the common baseline was to have 10 weeks worth of sales in surplus, in which case the PS4 needs around 2.5 million in stock to be normally shipped. Saying 1.5-2 million is a good number sort of ignores the differences in sales frequency - the Xbox One needs fewer units in stock than the PS4, likewise, the PS4 needs far fewer units in stock than, say iPhones.

Idk, maybe thats what it is, just seemed like the amount that various popular systems like Wii, 3DS, PS4, Switch have on shelves when there isnt shortages or overshipped.

But how do you figure what 10 weeks of surplus is when sales vary throughout the year?