You don’t need to earn a total of £18,600 in 9 months.
You just need to have been on a salary of at least £18,600 per year for a minimum of 6 months (so that will be at least £9,300 total gross income in those 6 months).
For Category A, it will be calculated as:
Total annual income that can be used for the visa = Lowest gross payslip x 12
As long as this amount is at least £18,600, you will meet the requirement.... which means that as long as none of your payslips is less than £1,550 before tax, you will meet the requirement.
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