" Why doesn't Turbo Tax just use the property assessed land value? That's what the IRS recommends. "
Assessed by whom? One thing the IRS does state, is that you do not use property tax values. You use the LESSER of what you paid for it, or it's FMV at the time it was placed in service. Generally, though not always, on rental or other business property, the lesser value will be what you paid for it.
TurboTax does use the property tax values to determine the ratio, and that's all. That's why the program asks for your property tax values. So if the property tax value is $100K for the whole thing, with the land valued at 30K, then the ratio is 30%. TurboTax then applies that percentage to what you paid for it, to determine the amount to allocate to the land. Now the IRS "does" recommend that.
