1040 line 42 question

If line 38 is over $155,650, then use the calculation table in the instruction. However, if line 38 is less than $311,300 (married filing jointly), using the Deduction for Exemptions Worksheet—Line 42, line 5 is negative, should we use the negative number in the calculation or use $0, if use the negative number in line 5, the final number for line 42 will exceed "Multiply $4,050 by the total number of exemptions claimed on Form 1040, line 6d". Please help. Thank you!