[Avodah] Open Door Policy

[Harvey Benton]

Sometimes a married woman finds herself in a situation where she may need to be secluded with a man who is not her husband.  For instance, if she visits a doctor or dentist, or has a repairman over her house to fix something.  From my understanding, the policy is to leave a door open so that others can walk in (or see in through a window?) at any time -- thus alleviating or obviating any potential yichud problems.  
 
My questions are:
 
1. Is there a minimum shiur, like a tefach, that a door has to be open?
2. If there is traffic on the street, and people can look in through the window to see activities in the house (or specifically into the room being worked on), would leaving the blinds open be enough? or does a door have to be actually open?
3. Are these comings and going halachas similar to kashrus halachas involving non-Jewish cooks, where the cooks have to be constantly in fear of a Jew entering, thus they will be careful to not do anything against halacha? 
4. What if someone lives in a totally secluded area, where there is no street traffic at all, would leaving the door open be any help at all?  Or do we say that there is always a possibility of a visitor coming, which would alleviate the problem?
Thanks much, HB
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