see form 8959.
shortcut
1) total of box 5 on all w-2s less $250,000. the net times 2.35%
2) add 3625 to line 1
3) total of box 6 on all w-2s
4) net additional tax subtract 3 from 2
example
total box 5 $600000
less $250000
net $350000
times 2.35%
result $8225
add $3625
result $11850
total box 6 $8700
$11850-8700= $3150 net additional owed
say total withheld was $10,000 so net additional owed is now $1850
then schedule 1 line 11 would be $3150 ($350,000 - excess over $250,000 - times .9%)
$1300 ($10000less $8700) will appear as additional withholding on line 25c of the 1040
