Estimated value of house

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I am currently working on a deal which involves a 4 bedroom, 2.5 bathroom and 2741 sqft. I am a newbie but I have been really good with my numbers in terms of ARV. I rather have my buyer profit and come back anytime to do business then accept a big payday. I value trust, honesty and being upfront over money so I take the ARV very seriously. After some comps I came to the conclusion this house was worth 320k. The comps were good but not perfect cause this house was special in size out of a lot of houses in a 5 mile radius. I tend to take only a mile radius, some cases are complicated so you push out. There is a house 4 miles away from my house that sold for 415k, it only contains 2 bedrooms and 2 bathrooms with 1,300 sqft. Then I said there must be something special about the house, an advantage or an amenity. It sits on 1 acre while my house sits on 0.63 acres, other then that there is nothing else. This house is not only much smaller with less bedrooms and bathrooms but was built in 1857, my house was built in 1936. A real estate agent told the owner of the home, her house is worth 400k or a little more. I'm stuck in the middle here, could be a good investment for my end buyer based on repairs and other factors if I offer her the right price. Someone please give me some advice, I look at my investors as people who help me so I do my all and best due diligence to help them.