Created the code to calculate using the new options, using the 2019 calculations per draft Pub 15-T. Using annual figures, all with 75k taxable, no extra income, credits, or deductions.
Single 2 Jobs (2020 W4 option) 13623 FWH
Head of Household 2 Jobs (2020 W4 option) 12175 FWH
Single 0 Allow (old W4) 11522 FWH
Single 1 Allow (old W4) 10598 FWH
Single 2 Allow (old W4 or 2020 W4 form with implied allowances) 9674 FWH
Married Jointly 2 Jobs (2020 W4 option) 9674 FWH
Married Jointly 0 Allow (old W4) 7196 FWH
Head of Household 2 Allow (2020 W4 option) 6901 FWH
Married Jointly 2 Allow (old W4) 6188 FWH
Married Jointly 3 Allow (old W4 or 2020 W4 form, implied allowances) 5684 FWH
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Note the two in bold. Shows insight as to how the Married 2 jobs was "built". Using the new W4, Married with the two job box checked is the same WH as Single (which has 2 implied allowances)
The Single 2 jobs is the "new" calculation, as well as federal Head of household WH status.
Using my test figures, the possible under withholding (new form) for those with less than the implied allowances, is significant. For those preparing the new form, knowing the implied allowance numbers will be needed, if comparing to knowledge of what was on the old form, or using "experience" with the old form. This can be handled relatively easy. Whatever the allowance value is (currently 4200), if you want to claim less than the implied number of allowances, add the allowance amount (currently 4200) per each allowance less you want to claim, other income (4a). To equal what the old Single 0, use Single, with 8400 (using current allowance figure) as other income.
With the new Single 2 jobs being radically more WH than the current Single zero, and with many set on the idea of using the current Single zero as a "fail safe", the new "fail safe" would be Single, 8400 (adjust for the new allowance figure for 2020 and beyond) other income.
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Of course, until the final revisions are released, it is only speculation... With no impetus to negate the existing forms, the calculations and W4 seem reasonable, even with the ugliness of implied allowances in the new calculation.