You didn't tell us but I suppose the gain from the sale of property is mostly, if not all, unrecaptured §1250, which got taxed at lower marginal rates because, as you said, your client is a 24% taxpayer.  On that basis, there's no tax rate differential to adjust for either Line 1a or 18.

You mentioned previously that there is an adjustment for Line 1a.  I think you may have mistaken.  There is a statement for Line 1a but it probably only shows what it consists of and there'd be no adjustment.