House across from a dairy farm

[Amy A.]

How much would you reduce the after repair value of a house across from a dairy farm?  I grew up working on such a farm and understand that there are smells and flies related to such a business.  However, I enjoy farms so am not able to look at it objectively.

For houses that are on a busy street, I reduce the value compared to comps that are on quiet streets by about 15%.  I have had great success with this strategy.  Is there a similar % reduction that I should use for being across from a smelly business?