The descent rate calculation states that descent rate is calculated by 'groundspeed / 2 * 10'.
Another document stated that the ground speed should be multiplied by 5 (or more accurately 5.2) to acquire the descent rate. The 5.2 value is found by finding the gradient for 3 NM for 1000 feet (based on the 3:1 glide ratio), which is 5.2% (found using tan of 3 degrees). The document can be found here: https://www.ivao.aero/training/documentation/books/SPP_APC_Top_of_descent.pdf
My question is: How was the formula created? It's interesting that I can't find any info on this on the internet. Can someone show the maths behind this formula?