Yes, just to add to the link.
The SLC do not work out an amount. The figure is fixed as a percentage over the threshold and gets deducted by the employer through PAYE and it appears on your pay slip as a deduction. The employer then declares the amounts paid back to HMRC in the same manner that tax and NI gets declated to HMRC, and they pass the info back the SLC, who deduct the repayment amount off the debt. This goes on for several years until the loan is nearly paid off, at which SLC finalise the last payments with the employer and advise that no further deductions need to be made.