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[kitz]

I never knew it was synch speed they were predicting. 

Yep that actual line rate

Downstream Line Rate(Mbps) Up to 62.6
Upstream Line Rate(Mbps) Up to 20

Downstream Line Rate(Mbps) Up to 17
Downstream Range(Mbps) 10 to 19.5

Throughput/download speeds will be less than line rates and can be affected by a number of factors within and external to BT's network, Communication Providers' networks and within customer premises.

Dunno when it changed to 17, was still 16 a couple of months ago when I ordered vdsl.
   

[kitz]

Yes, I realise it is not scientifically ideal, bur w/out recruiting an army of experimenters it's all I can do on my own.

Good luck 
You may find  the communities may help.
I know when I first wrote it, there were quite a few from these forums/PUG/PN/Be*/TBB who did compare their results and it generally was fairly accurate with its prediction.
Ironically it still seems pretty acceptable even if using the adsl2+ line attenuation.

There could be a few reasons for that.. despite the adsl1 figure may give you a few kbps more... 
1) The early testers were the most likely to have maximised their line to its full potential.
2) Crosstalk - which is there now on most adsl2+ lines.   
- When I went over the Be, I was one of the first on their shiny new MSAN, my order had been placed before the live date and as mentioned I got the full 24Mb/2.6Mb.. after a few years when they put all the o2 users on, it slowly started to decline with the tell-tale slight curves across certain tones, increasing up again after tone 512.  I started to lose more speed over the past year.. but that could well have been due to a line fault starting to rear its head, that only got fixed a couple of weeks ago when it also started affecting voice. 

[XRaySpeX]

I appreciate that this is a sample of one ...

My BTw based WBC line, using a tweakable Netgear DG834GT router, syncs on ADSL2+ at:

• 19.0 Meg normally @ 3 dB Target NM
• 17.2 Meg tweaked @ 6 dB Target NM (DLM didn't complain}

Its attenuation for ADSL2+ is 26 dB but for G.DMT is 23 dB as you would normally expect.

Now your Speed Calculator predicts ADSL2+ speeds @ 6 dB Target NM of:

• 19.1 Meg @ 23 dB attenuation
• 17.6 Meg @ 26 dB attenuation

Therefore on a short to medium length length line, to get the closest prediction for the ADSL2+ speed at 6 dB Target NM, it would seem best to input the ADSL2+ attenuation or, if this is not yet known, the G.DMT attenuation + 3 db (as the expected rise in attenuation).

EDIT: Perhaps it is interesting that using the G.DMT attenuation gives a good prediction of the ADSL2+ speed at 3 dB Target NM.

Of course, this effect may tail off on long lines and it would be interesting to see a similar analysis from lines greater than 50 dB, say, attenuation that are capable of currently running ADSL2+.

[kitz]

I appreciate that this is a sample of one ...

Thanks for sharing the info.  I still have some of my old line stats, but with being on a short line & annex_m they arent much use to prove or disprove anything.  I did at one point switch to annex_a when I was messing with a ST585 but I could easily sync at 24Mb on Annex_A anyhow. 

With hind sight I should have tried it before I went fttc to see how much the upstream was costing me, but my line was starting to degrade then so not sure how accurate any tests would have been. 

it would seem best to input the ADSL2+ attenuation

As mentioned in my post above the adsl1 was pretty accurate in 2007, now it seems to be adsl2+ for some reason.  I do think crosstalk has had a big impact on attainable speeds.  I know I had a line fault towards the end, but before that and over a period of 6yrs, Ive slowly seen my line degrade by about 2Mbps due to x-talk