What is the maths behind the descent rate calculation?
From Gypaets' answer
$$DescentRate = 0.0524 \cdot GroundSpeed$$
But the Ground speed is in nautical miles per hour [NM/h], and the descent rate is in ft/min.
So we have
$$DescentRate \left[\frac{ft}{min}\right] = 0.0524 \cdot GroundSpeed\left[\frac{NM}{h}\right]   $$
With $6076.12 \left[ft\right]$ in 1 $[NM]$ (yay for imperial units) and $60$ $[minutes]$ in 1 $[h]$ we get a conversion factor:
$$ 1 \left[\frac{NM}{h}\right]= \left[\frac{6076.12~ft}{60~min}\right] = \frac{6076.12}{60}\left[\frac{ft}{min}\right] \approx  100\left[\frac{ft}{min}\right]$$
This leaves:
$$DescentRate \left[\frac{ft}{min}\right]  \approx 0.0524    \cdot GroundSpeed \left[\frac{NM}{h}\right] \cdot 100_{(NM/h~to~ft/min)} $$
Further approximating it to:
$$DescentRate \left[\frac{ft}{min}\right]  \approx 5 \cdot GroundSpeed \left[\frac{NM}{h}\right]  $$