You can't just use any method that you like. The tax law gives you a choice of two methods to determine your deduction for sales tax. One method is to use the actual amount you paid, as documented by your receipts. Adding up your receipts is the only method allowed for determining the actual sales tax you paid.
The other method is to use the Optional State Sales Tax Tables that the IRS publishes in the Instructions for Schedule A. The TurboTax calculation uses the IRS sales tax tables. The IRS Sales Tax Deduction Calculator that DoninGA referenced uses the same IRS tables.
Those are the only methods allowed for calculating your sales tax deduction. You have to pick one or the other.
