What do people consider a stable SNR?

[#1]

I asked for the target SNR on the line to be dropped to 6dB, this resulted in sync speeds of around 3500kbps, however the line was pretty unstable (from what I could tell at least a couple of drops a day), so it's now been moved to 9dB. It is now syncing around 3000kbps, but did drop this morning. The router is a Netgear DG834G, it was pretty much 100% stable at 15dB, but gave sync speeds of only 2000kbps.
Is there any hard and fast figure that's considered stable? It would seem this is 15dB, but I can't think that's the same for every line/router combination.  I was hoping 9dB was stable, but it looks like I may have to drop to 12dB (I don't want to carry on messing PN around if this isn't going to be stable either).

[#2]

Unfortunately there's no hard and fast figure, it all depends on line length and quality though there is a direct relationship between SNR and sync speed - if you drop the snr, the sync speed will go up but the service will be more susceptible to interference.
Hope that helps,

[#3]

I have asked for my snr to be lowered but F9 refused to do it
My line length is only 350 feet
Come on F9 please put in the request in
Puddy
(account=jillweekes)

[#4]

Hi Puddy,
As per your ticket, I've raised a request with BT to amend your target SNR.