IHTM26108 | Step 3 - interaction: the appropriate fraction
From HM Revenue & Customs · Inheritance Tax Manual
All specific gifts other than those of relievable property are reduced proportionately by using the appropriate fraction at . The fraction can be expressed in two alternative ways depending on whether there are
specific gifts of relievable property (), or
no specific gifts of relievable property ()
The fraction is based solely on property at the title () you are concerned with, normally the free estate (). Property chargeable at any other title, such as settled property (), is excluded.