Whats in the life interest trust?

W died leaving a Will with a nrb trust and residue on a life interest trust for her spouse H. No assets have yet been appropriate to either trust. For IHT purposes, we need to calculate the value of the life interest trust as at the date of H’s death. How is this calculated as the trustees can select what assets they wish to transfer to the nrb/life interest trust but they have not yet done so?

Further, one option is to transfer a solely owned property worth say £500,000, partly to each trust, say £325k to the NRB trust and £175k to the life interest trust. Will the value of the life interest trust then be eligible for a discount?

Is there any other consideration that I shoudl be aware of, presently the main point is to value the life interest trust.