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HMRC internal manual

Inheritance Tax Manual

Reduced rate for gifts to charity: abatement: caused by grossing up

The value of the chargeable transfer cannot be more than the value of the estate for Inheritance Tax purposes, so where the grossed value of the estate exceeds that value, IHTA84/S37(2) applies, with the result that the grossed up value is reduced to the value of the estate as described at IHTM12088. You should use this value of the estate to calculate the baseline and apply the 10% test.