Not a lot of use in these circumstances (where salary for only eight months of the year is expected) fadeout.
If you use an annual figure of £11,333 (which is correct) the calculator gets the annual tax correct (though it gets the monthly figure wrong because it assumes 12 pay packets). But it under estimates the NI deductions (because NI is levied monthly regardless of the total annual income).
If you use an annual figure of £17,000 the monthly NI is correct (though the annual figure is wrong) but the tax is completely wrong (because £17,000 will not be earned).
The moral of the story: do not blindly rely on calculators :-)