Glad it worked!
I think your ISP won't set this bit so rather have ipfw match on it being 0. Otherwise, excluding those last two bits from the match might be more correct:
I haven't delved much into DSCP, but it looks like those last two bits aren't used. My guess is your ISP will either set them to 0 or not modify them from what they receive from upstream.
Now, i don't understand, why mincost bit need to be 0? Or when need to be 0 or 1?
ipfw add 20 count ip from any to any ipprecedence 1 iptos \!lowdelay,throughput,\!reliability