How to calculate your subsidy:
1) Figure out your income as a percentage of the poverty level. For example $125k is 356% of the
five-person poverty level used for ACA healthcare in 2024.
2) Find the "applicable figure" for that income in the
lookup table for Form 8962. For example this figure is 7.40% for someone at 356% of the poverty level.
3) Multiply this figure by your income. $125k * 7.4% = $9,250, or $770.83/month.
4) Look up the unsubsidized cost of the second-cheapest silver plan in your area for your family members who will be buying Exchange coverage. For example when I search for plans in King County for a couple of 40-year-olds with 8-year-old triplets, I find the second-cheapest silver plan is "Ambetter Cascade Select Silver" at $1,515.97/month.
5) Subtract the figure from Step 3 from the cost in Step 4. $1,515.97 - $770.83 = $745.14. This is your subsidy amount that can be applied to any plan (with the caveat that plans with unsubsidized cost less than $745.14 will just go down to zero; you won't be paid to take them).
Incidentally when I put an income of $125k in for this example family in the "Browse and Compare plans" interface on the WA health plan finder site just now, it gave me an estimated tax credit subsidy of $745, exactly the same as in the calculation above. What are you seeing differently that causes you to doubt the site's accuracy?