I'm not an IFR driver heres my view of things for those who care
The Maths:
TOA
tan(3)=ROD/GS
tan(3)*GS=ROD (knots)
Theres 1852 m in a nm, and 3.281 ft in a m so nm > ft is *6076.412 and of course hours to minutes is *60.
So converting knots to fpm is 6076/60 = 101.2735
tan(3)*101.2735*GS=ROD
tan(3)*101.2735 = 5.307
So all that crap means that for a 3 degree approach GS*5.307 gives required ROD, so why is 50 ft added? possibly to account for the .307 but then again could be wrong.
My 2c