It is tag Baseline Noise (hex. C62B) - see dng specification. I lowered value of this tag from 300/100 to 200/100 in A620 template of DNG4PS v1.14 (I'm still using that version, bacause i think it's still not possible to customize matrix in DNG4PS2, and I want to use my calibration). Also changed other tag - Baseline Exposure - to value 0/100 (it was +50/100)
From my remembering, in dng made from EOS 400D raw file, baseline noise is 110/100 and baseline exposure is -25/100 (that's why DSLR-s have some exposure headroom in highlights - if jpeg is correctly exposed, raw is actually underexposed by 1/4 stop, and ACR takes this into account)