Pay for your own, independent appraisal. Call several Real Estate agents to get recommendations on expert appraisers.
The first place to start is to get a handle on what other like properties or square footage is appraised at in your area. If everyone is appraised at ridicules levels, it will show you that a comparison approach will not work.
After that, determine what value your property is supposed to be assessed to...fmv; replacement values; costs plus yearly inflation or whatever.
That is how I would do it were it me. Our tax basis here is original sales price plus an allowed 2.5% per year for inflation assuming nothing changed like adding a pool or more sq footage. It works fine for owner occupied dwellings. Non owner occupied not so good.