Intertnationally mobile employee

[Brightonian]

I have been asked to complete a tax return for someone who was granted equity in March 2022 which vested in March 2025. He was UK resident throughout but was also resident in the Netherlands and Germany in 2022 - 6 months in each country. His employer has sent me the relevant Dutch and German payslips and I shall show those amounts on separate employment pages of the 2025 UK return.
My difficulty is that I am unsure about the calculation of FTCR. I believe that the amount on which relief can be claimed against UK tax is based on the number of workdays between the dates of grant and vesting and that it is restricted to take account of UK workdays during this period. However, I can find nothing to confirm this method of calculation. I should be grateful if someone could point me to some guidance in this area.