Overtime is not a separate tax. It is extra wages. On Takehome, regular hours are paid at the hourly wage and overtime hours are paid at that wage times the multiplier. The default multiplier is 1.5, which is time-and-a-half. You can change it if a contract uses a different rate.
A weekly example
Someone paid $20 an hour for 40 hours earns $800 gross. Ten more hours at time-and-a-half are 10 × $30, or $300. The check is $1,100 gross before anything is withheld. Because the estimate assumes that larger check repeats all year, the federal tax on an overtime-heavy week will look higher than the tax on a normal week.
Hours belong to one paycheck
Enter the hours for the paycheck you are looking at, not the hours for the whole month, unless you are paid monthly. A weekly full-time schedule is about 40 hours. Every two weeks is about 80. Twice a month is about 86.67, and monthly is about 173.33. Mixing those up is the most common way to get a take-home number that cannot match a stub.
Some jobs pay daily overtime, double time after 12 hours, or a different rate on Sunday. The multiplier field is there for that. The calculator will not apply those rules by itself.