USB camera driver - intelusb example in Windows DDK

Dear All,
I am writing usb camera driver. I went through the intelusb example given
in the DDK and got totally confused by the way Microsoft is calculating
the Average time per frame which is being passed to KS_VIDEOINFOHEADER.
The author has directly put the magic number there without any information
about from where he got it. What I could understand from the example is
that they have taken QCIF as base and then they do some jugglery in the
CamAllocateBandwidth to convert other resolutions to QCIF by multiplying
them by 2/3 to obtain the 160*120 resn. and by 3 to obtain the 320*240
resn and then compare it with the values passed to KS_VIDEOINFOHEADER
structure. Can somebody please explain me as to what is the rationale
behind all this calculation and what does a magic number such as 416666 or
1666665 means…