MF_PD_DURATION return imprecise value

Question

Hi I've created a simple decoder for windows 8.1, the output format is pcm_wav header 46bytes long.
Ok the class works well and writes a file .wav well formatted.
The problem is the length of pcm data, I calculate at the beginning of process
in this way:

HRESULT GetSourceDuration(IMFMediaSource *pSource, MFTIME *pDuration)
{
    *pDuration = 0;

    IMFPresentationDescriptor *pPD = NULL;

    HRESULT hr = pSource->CreatePresentationDescriptor(&pPD);
    if (SUCCEEDED(hr))
    {
        hr = pPD->GetUINT64(MF_PD_DURATION, (UINT64*)pDuration);
        pPD->Release();
    }
    return hr;
}

this return (100ns ticks) an odd value es: 2593440625,
from this I calculare the length of pcm in bytes

(2593440625 /1e7)*(nChannel(2)*nByteIntoSample(2)*SampleRate(44100) )= 45748292,625
I already get odd thing , 625 (??)

by the way the right size of pcm is 45748224, that you know after write all bytes,
yep the difference is only few bytes but the point is that: MF_PD_DURATION get wrong time, or imprecise time.
So the question is how to get right pcm byte length?

thanks.