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@HOPE2 wrote:

However, when I enter this amount on line 18 for SE tax, the result of $240 on line 26 does not seem to appear on Schedule 3, line 12e.

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There is another worksheet you need to go to. 

It is on the 1040Wks (not the actual 1040, but the 1040 Worksheet), then scroll down to Schedule 3 and there is another worksheet.