All I do is remember 3 x dist in nm;
i.e. 5nm out 3 x 5 = 15 therefore 1500ft (AGL)
or 4nm out 3 x 4 = 12 ---"--- 1200ft
Use the same rule for straight in visual approaches (remember field elevation if significant) when using QNH.
Also works well if you want to do a constant descent visual approach if high above an airfield. You may declare and be cleared for the visual when high on the profile (if you were going past the field to join the ILS at 8nm and you declare visual you will now be high on a visual approach (obviously you don't want to fly all the way out to 8nm and then come back to land) so how do you decide when to turn base and final?
What I do in the 757 is position to end up a couple of miles out from the field on a downwind with the gear down and flap 20 set. If very high you can take speedbrake as well to really get the rate of descent up. If there is a DME on the field (or a fix in the FMC) that gives you distance out, so for example, as I pass 3 mile downwind I use the same rule which will give me 900ft. Well that's what I should be inbound so I add 1000' for the base and final turn giving a target at 3 miles downwind of 1900'. If I'm at 3000' then it's too early to turn in so I keep going downwind and calculate for 4 nm (2200'). If I'm 'in the ballpark' (say within 500') I can start to turn base, take flap 30 and adjust my rate of descent to give me my 3 degree slope (always remembering to keep an eye on the wind).
For the 3 degree glideslope I use another rule of thumb;
Drop the last zero from the groundspeed and halve the answer which will give you your required rate of descent in 100's of feet per minute;
So, G/S 140kts means 14/2=7 = 700fpm
which I find a lot easier than 5 x groundspeed which works as well.
Sorry if you knew all that, hope it's useful to others.
PP