Worked example: payor $201,000, recipient $29,640, 3 children, Box 1, child care $300/wk paid by recipient
Base order (no child care)          $1,013/wk  $52,662/yr

1.  CURRENT (Line 3c, pre-transfer)       payor funds  87.7% of child care (+$13,678/yr; order $1,276/wk)
2.  POST-TRANSFER GROSS shares            payor  64.3% -> funds $10,033/yr
2b. POST-TRANSFER on Line 3a (redline)    payor  64.5% / recipient  35.5%  -> funds $10,054/yr; order $1,206/wk vs $1,276 (saves $3,624/yr)
3.  POST-TRANSFER NET shares              payor  49.7% / recipient  50.3%  ($90,447 vs $91,511)  -> funds $7,754/yr
4.  POST-TRANSFER NET incl. child care    payor  49.7% (fixed point; equals rule 3)
5.  POST-TRANSFER NET, withholding basis  payor  53.0% / recipient  47.0%  -> order $1,171.64/wk vs $1,275.77 (saves $5,415/yr)   <-- THE ASK
    payor net $139,834 (30.4% effective), recipient net $24,734 (16.6% effective)

Pre-transfer gross share (Line 3c basis, approx): payor  87.1%
Caveat that travels with every line: per person the payor still leads ($90,447 for one vs $22,878 each for four, rule 3).