Don't want to nit pick but as far as i know, most DME are at the VOR position arent they, which is not at the threshold, so that wouldnt be super accurate would it? Would give a reasonable indications though id imagine.
Yep, happy to back up richo with the same rule of thumb that we use at my abode and also where I use'd to teach.
If -
@ 6nm from the threshold : 1 dot +/- slope = 100 ft
Each nm either side of the 6 nm position, 20 ft can be +/- from the deviation.
ie @3 nm from threshold -: 1 dot deviation = 40 ft (100 - 60).
ie @9 nm from threshold -: 1 dot deviation = 160 ft (100 + 60).
Hope this helps
PS dunno if it's true - might have to wander off the glideslope one day