Tenatative Business Income

On Form 1120S, Depreciation Options, Line 3A for Section 179 depreciation lists 'Tentative Business Income' and posts a number that is considerably higher than my actual net imcome and this causes TurboTax to calculate my 179 depreciation incorrectly, causing my TurboTax net income to be a loss.  Section 179 depreciation requires a rollover for any amount that puts you in a loss.  Any ideas?  Thanks!