Calculating PAYE manually

Hello, Although we use Sage Payroll to calculate PAYE, I have been trying to also calculate it manually, but can't understand where the figures have come from. Using the manual tables: https://www.gov.uk/government/publications/taxable-pay-tables-manual-method, looking at Table C1 for Monthly paid employees, I see Column 1 is £3142, which is the monthly threshold for a Basic Rate taxpayer (£37,700, divided by 12 months, rounded up). But I can't understand where the £628.46 in Column 3 comes from? I'd have presumed 20% of Column 1, but doing that doesn't work, using either £3142, or the exact £3141.66. Could anyone explain please? Thanks.

Hi,

There will be slight rounding differences which will catch up with itself in future months.

Thank you.