Sounds like like math errors, so you can't use 3115 to correct. What you need to do is correct the basis. Your basis is reduced if you took the wrong amount of depreciation on your tax return. If you claimed too little depreciation, you must decrease the basis by the amount you should have taken. If you took too much depreciation, you must decrease your basis by the amount you should have deducted, plus the part of the excess you deducted that actually lowered your tax liability for any year.