I've learned (at least I think this is true) after reviewing the IRS instructions for Form 1116, that what (and how) you use Line 18 is related to what (and how) you use Line 1a. See pages 10 & 11 along with pages 23 & 24 of the IRS instructions for Form 1116.
I qualify for the "adjustment exception", but as I understand it, these exceptions are taken as a pair for lines 1a & 18, or not at all. This (presumed fact) leads me to be better off by not using the worksheet for line 18.
None of this answers how one would invoke the Worksheet for Line 18 if a user would be better off using it.
TaxDavey.
