You need to have many "comps" to prove your value. Use other properties that are close in square footage, the assessors are big on per square foot value's, lot size and value that at per square foot also. Year built as compared to others and remodels if any compared to others.
We have a motto in our county, do not let the assessor come into your home, when they make their visit. If you have done any remodeling, like upgrading the vanity in a bathroom or upgrading kitchen counters with granite, if they see those they will put a higher value on your home.
One last thing, if you have ever put an over the top price on your home for sale and it didn't sell, they will use that against you and say you listed your property for such and such, that price is obviously what you think the value of your home is, I saw this repeatedly in this years meetings.
Real estate values here are at least $100k over the assessed values on your average 3 bedroom home.
I’ve been calculating sq ft values and they are all over the board. I’m seeing them from $80 to $140 for the same siding construction and built about the same time.