How do I assign the land value vs improvement value of my rental property based on the gross assessment on my real estate tax?

Your tax assessment should show the valuation they used for the land and the "improvements" or building separately.  If not, look it up on the tax assessors web site - it's public record.  Calculate the percentages they use, and then apply those percentages to your purchase price.  The TurboTax interview will guide you through the process.