according to quick google then you don't need to pro rata. it's just flat £6 regardless https://www.att.org.uk/home-sweet-home-%E2%80%93-tax-relief-home-working
The weekly, flat-rate amount applies equally to part-time workers and it is not necessary to pro-rate it because the employee does not work at home full-time.
gov.uk has a page to check quickly https://www.gov.uk/tax-relief-for-employees/working-at-home
Thanks for indulging my laziness!
@hoefla started
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according to quick google then you don't need to pro rata. it's just flat £6 regardless
https://www.att.org.uk/home-sweet-home-%E2%80%93-tax-relief-home-working
gov.uk has a page to check quickly
https://www.gov.uk/tax-relief-for-employees/working-at-home