Measure scalloping loss of window function
Harris defines the scalloping loss as the ratio of coherent gain for a tone located half a bin from a DFT sample to the coherent gain at a sample (see the paper for the definitions)
$$
\mathrm{SL} = \frac{\left\lvert \sum_n w(nT)e^{-j\frac{\pi}{N}n} \right\rvert}{\sum_n w(nT)} = \frac{W\left(\omega_{1/2}\right)}{W(0)}
$$
Where $w$ is the window and $W$ is the window DFT

$$
\omega_{1/2} =\frac{\pi}{NT}
$$

Guess the value you are after is really $1-SL$

EDIT: I believe the left hand side of the equation is enough to answer @Shaggi's question, but to clarify a bit the notation of the rest, Harris defines DFTs as functions of $\omega_k$, so

\begin{align}
  W(\omega_k) &amp; = W\left(\frac{2\pi k}{NT}\right) = \\
              &amp; = \sum_n w(nT) e^{-j\omega_k nT}\\
              &amp; = \sum_n w(nT) e^{-j 2\pi kn/N}\\
  W(\omega_{1/2}) &amp; = \sum_n w(nT) e^{-j \pi n/N}\\
  W(0) &amp;= \sum_n w(nT)
\end{align}


  F. J. Harris - On the Use of Windows for Harmonic Analysis with the
  Discrete Fourier
  Transform