Sorry, but I really think it is suspect.
Granted my line may be exceptional
, I put in the Kitz figures, which I understand is meant also to cater for mid-range expectations, for my 26 dB attn. line: 17600 K, 26dB attn., 6dB NM and get the results:
Estimated connection speed using ADSL2+ mode (maximum 24 Mbps)
Normal speed range at 26dB attenuation is 15000 Kbps to 18000 Kbps
Target Margin Estimate
This predicts a fantastic +/- 5 Meg gain of speed per -/+ 3 dB delta in NM. Furthermore it predicts my actual speed @ 3 dB NM as slightly below its estimate and so can no longer be regarded as exceptional.
I would not expect them to be identical, but I would be extremely surprised that 2 reputable estimators, yours & Kitz's, would produce vastly different results.
Sorry again, but I think you really need to look at it.
1999: Freeserve 48K Dial-Up => 2005: Wanadoo 1 Meg BB => 2007: Orange 2 Meg BB => 2008: Orange 8 Meg LLU => 2010: Orange 16 Meg LLU => 2011: Orange 19 Meg WBC