In a nutshell, the more you 'push it' the higher the error count your modem will see. Unfortunately, the error limit at which the DLM will decide that your line is unstable and increase the target SNR by 3dB to stabilise it is not documented, or publicly available. It is also very likely that the error rate at which the DLM increases the SNR is different between different DLM chipsets and modem combinations.
From my experience the absolute FECs, CRCs counts is not important - during thunderstorms I had FECs increasing to thousands - but the rate of their change is critical to the DLM kicking in. The sensitivity of the DLM to line error rates is determined by the 'service setting' (extra stable, stable, standard) that BT have applied to your line the exchange. Have a word with your ISP if you want to change this, but bear in mind that with your distance from the exchange you may alsready be at your best setting anyway.
Another thing to pay attention to is REIN. Your line stats look nice and steady until REIN occurs and then suddenly you lose weeks of 'good behaviour' and get a penalty from DLM.