Inheritance Tax - Transferring Thresholds from more than one spouse

hoof

I would appreciate clarifaction on transferring unused nil rate band thresholds where someone has re-married based upon the following:
Anne died April 2007 leaving estate worth £200k to her husband James. The nil rate band threshold at this time was £300k. As there were no legacies, my understanding is that 100% of her nil rate band was unused and thus available to James on his death.
James then marries Joan in 2010. James, unfortunately, dies in late 2012 leaving an estate of £260k to his children from his marriage to Anne (nothing left to Joan who has independant means and in no need of any of his estate). Nil band threshold at time of James death is £325k.
Presumably James has total nil rate band threshold of £650k (100% unused re Anne and his own = 2 x £325k), with the legacies of £260k to his children from marriage to Anne leaving £390k unused (120% of nil band threshold).
On Joan's death, will the nil rate threshold for Joan's estate be 2 X nil rate band at time of her death (£650k using current rates) - representing unused balance transferred from James (capped at 100%), plus her own allowance?

Would appreciate feedback as to whether my understanding is correct. Many thanks in advance.

xylophone

http://www.hmrc.gov.uk/manuals/ihtmanual/IHTM43035.htm
The relevant guidance?

hoof

Thanks for that link - just what I was looking for. I had come across various examples on hmrc site but they were not quite what I was looking for. Seems that the key, in case I outline above, is for James personal representatives to make a claim to transfer the unused nil rate band from Anne's death to maximise nil rate band available for Joan.