The tax bill value is not used on your tax return at all. The program only uses tax values to determine what percentage of your cost-basis is applied to the land value. That's it.
If your tax bill shows a total value of $100,000 and a land value of $25,000, then 25% of the total value is allocated to the land.
So if you paid $300,000 for the property, then 25% of that $300,000 is allocated to the land.
In the program you would enter $300,000 in the "COST" box, and enter 25% of that ($75,000) in the "Cost of Land" box.
This is assuming that what you actually paid for the property when you originally purchased it, was less than the Fair Market Value at the time you placed the property in service. Now-a-days, it is more common for the price you paid to be lower than the FMV.
