I'm not sure of what an acceptable amount of continuity errors would be.
You did say that at times you see a total muxed output reach 40Mbit. I would try to get that down to around 37-38 ish to start, because it will tile with 40. I generally run 2 HD's into a 256QAM with maybe 1 SD without any problems. 
As far as errors off air, I try to have none and sometimes that means installing a new antenna or doing a repoint. 
This site has some useful info.
http://www.broadcast.net/~sbe1/8vsb/8vsb.htm
Good luck 
    
    Re: Acceptable Error Counts
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