Calculating the extent of spectral leakage
Windowing in the time domain results in a convolution with the transform of that window in the frequency domain.  So you need to compute the result of this convolution to see the effect (which some call "leakage").

You can sample the transform of the window function used (rectangular, Von Hann, etc.) to determine the FFT frequency response of a windowed pure single frequency sinusoid.  For a rectangular window, the magnitude of the apparent "leakage" in an FFT result will be samples of a normalized Sinc function (more precisely a pair of normalized periodic Sincs or Dirichlet kernels), sampled at integer offsets from the frequency bin center offset, multiplied by the sinusoid's magnitude.

An attempt at the equation for the full Dirichlet pair frequency response result is on my dsp web page here: http://www.nicholson.com/rhn/dsp.html#4 

But using just a normalized Sinc as the window transform equation is usually close enough, except for the bins right next to DC or 0 Hz and to bin N/2.

For windows other that rectangular, use their transform instead of a Sinc function, up-sampled and/or interpolated if needed.