Calculate a 20% tax on Gift Aid donations.
Check what is the total tax due before FTCR.
From a total tax due before FTCR, deduct a tax calculated on Gift Aid, that will give you the maximum amount of FTCR allowable.
Overwrite automatically calculated FTCR amount in with a value from the previous step. Check if the test in live is successful.
If still unsure, contact support.