Every number below is worked by hand from the Schedule 1-A Part III rules, and the overtime calculator reproduces each one.
Example 1: the case everyone gets wrong
Regular rate $30 an hour, 200 overtime hours in the year, all at time and a half. Total overtime pay: 200 x $45 = $9,000. Qualified premium: 200 x $15 = $3,000. The deduction is $3,000, not $9,000. If a calculator or a headline tells you the whole $9,000 is deductible, it is counting your regular pay for those hours as premium.
Example 2: paystub total only
Your year-end stub summary shows $9,000 of overtime pay, all at time and a half, and no premium line. Premium = $9,000 / 3 = $3,000. Same worker, same answer, which is the point of the division method in the paystub guide.
Example 3: double time
A utility worker is paid double time for 100 weekend hours at a $30 regular rate: total $6,000. The premium inside is $3,000, but only the FLSA-required part qualifies, one quarter of the total: $6,000 / 4 = $1,500.
Example 4: the phase-out
Single filer, premium at the $12,500 cap, MAGI $165,000. Excess over $150,000 is $15,000, which is 15 full thousands. Reduction: 15 x $100 = $1,500. Deduction: $12,500 - $1,500 = $11,000. Note the rounding: a MAGI of $151,700 gives an excess of $1,700, which counts as one full thousand, a $100 reduction, not $170. Part III drops the fraction.