, I'm Keith and happy to help you with your question.
Each deceased person has 325K of tax free estate before Inheritance Tax (IHT) kicks in. You are referring to the ability of a remaining spouse to add her deceased half's unused part of their 325K exemption from IHT to their own exemption limit.
Your Father will have will have used none of his 325K as inter spousal bequests do not count purposes, so a further 325K would be available to add to Mum's exemption giving 650K available on her demise. This 650K is, of course, inflated by any charitable and certain other bequests in her will. IHT is levied at 40% flat rate on any surplus over the 650K, but if charitable bequests exceed 10% of the estate the rate drops to 36%.
There is a further concession of which you may not be aware, that is Quick Succession Relief. Here is Caeser and Howie, The Central Scotland Law Group summary:
'Quick Succession Relief If a person inherits an asset on the death of another and then dies within a short space of time that asset will suffer IHT again on the second death. Quick Succession Relief mitigates this double taxation by giving a reduction on IHT payable on the asset on the second death. The amount of relief of credit is100% if second death occurs within 1 year of first death with a sliding scale relief up to a maximum of 5 years.
1 year or less 100%
1-2 years 80%
2-3 years 60%
3-4 years 40%
4-5 years 20%'
I do hope I have shown you how IHT works in practice.