Where to add tax credit values
What jurisdiction are you in? Countries typically have their own requirements for issuing Tax Receipts for Eligible Donations. 

In CiviCRM the Financial Type determines whether a Contribution of that Financial Type is deductible for income tax purposes or not. A contribution with multiple financial types can then become partially Tax Receiptable -> e.g. Meal$ (not eligible) + Donation$ (eligible). 

The non-deductible amount field (hit Edit Contribution to find it) -> is handy if you need to make manual adjustments.

Of course it still depends on the Code you use to generate the Tax Receipts how exactly it processes the line-items and the non-deductible amount field. We do that in our Canadian CDN Tax Receipts Extension. 

Hope that helps you along.