RAT 5,
I asked for angle of descent information; I could have asked for feet per NM, same thing. This is nothing to do with FPV.
Give or take a few feet, an aircraft descends about 100 ft/NM/degree. So on a 6 degree descent that would be about 600 ft/NM.
If at a given configuration the aircraft descends at 600 ft/NM, my calculation for deciding when to turn onto final from a 1.5/2 NM abeam downwind is as follows; distance from threshold in NM x 600 + Elevation. If my altitude is at or below this figure I turn onto final.
As an example, take a position 8 NM downwind for a sea level airport. 8 x 600 = 4800. If you are at 4800 ft or lower at this point, turn onto final. By the time you roll out you will be at 8 NM and 3600 ft. At 7 NM 3000 ft, 6 NM 2400 ft, 5NM 1800 ft, 4 NM 1200 ft, select landing flap and you will be stable between 500/1000 ft.
If the aircraft descends at 500 ft/NM, use 500 x NM + elevation, and so forth for different descent rates.
Any headwind on final will add to your descent angle, and if you feel it's a bit tight you could always select landing flap earlier.
What I like about calculating a descent this way is that you could be in a clean configuration as you pass abeam the threshold, and when cleared visual, establish the configuration and start calculating your NM/Alt. You don't need to time from abeam the threshold, and it's very satisfying to switch off all the automatics and fly the aircraft.
It also works for a straight in approach, you just have to reduce your altitude calculation by 1000/1500 ft to meet stabilised criteria.
I hope this helps to clear things up.