Does any one know how Line 28 on form 1040a, is calculated? It reduced on my additional child tax credit on form 8812.

This is the tax from the IRS Tax Tables determined by your taxable income on Form 1040A, line 27.  If you have tax, the Child Tax Credit, which is nonrefundable, first reduces your tax to zero before the Additional Child Tax Credit (ACTC)  is computed, which is the refundable credit.