How do you apportion the tax liability between two estates?

Malcolm, thank you this is v. helpful. Would this “formula” however still apply if say the ‘free estate’ of the deceased were only £500,000 (and therefore within the NRB/RNB)? i.e. would IHT still be payable from the free estate or borne solely by the trust?

I am looking at IHT against free estate - #2 by paul where the residue is exempt due to spousal exemption, but cannot find guidance / authority on whether a parallel is drawn where the ‘free estate’ is within the NRB (so no chargeable estate and only chargeable by virtue of the trust being aggregated).

Many thanks in advance