Huge amounts of small-hours experimentation would seem to indicate that for my Firebrick on a slow 20CN ADSL line, a suitable fudge factor for upstream is 0.888 (numbers in the range 0.880 - 0.895 all have their merits, values as high as 0.925 can push the download speeds slightly but really ruin the upstream speed which is the point of the entire tuning exercise). Take the upstream raw _sync rate_ given out by the modem, multiply by the above fudge factor and that is the number you out into the traffic shaper speed parameter in the Firebrick's config.
Are assuming that this converts from raw sync rate to bits per second of AAL5 SDUs. That is _everything that is sent to the modem_, eg. L3TPDU= IP header, IP payload and all.