First-Order Hold Filter Gain
That's correct, $0.4053$ is the approximate magnitude of the frequency response of linear interpolation at the Nyquist frequency. With the definition:

$$\operatorname{sinc}(x) = \frac{\sin(\pi x)}{\pi x}$$

the exact value is:

$$\operatorname{sinc}^2\left(\frac{1}{2}\right)= \left(\frac{\sin\left(\frac{\pi}{2}\right)}{\frac{\pi}{2}}\right)^2 = \left(\frac{1}{\frac{\pi}{2}}\right)^2 = \left(\frac{2}{\pi}\right)^2 = \frac{4}{\pi^2}$$

or in dB:

$$20\log_{10}\left(\frac{4}{\pi^2}\right) = 20\frac{\log\left(\frac{4}{\pi^2}\right)}{\log(10)}\approx-7.844795081\text{ dB}$$