The correct way to determine taxes is calling up the tax assessor/treasurer and ask how they'd calculate property taxes based on a certain sales price, assessed value, etc. Lenders usually go about it in a different manner though, they will either use the existing property taxes, will use a standardized percentage of the sales price (in the case of California it's 1.25% of the sales price), or will even go as far as asking the title company to compute property taxes (called a 'tax cert') on the new sale.
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