Your previous postings suggest you are pretty knowledgeable on a lot of this stuff, so I'm not sure I can see what nuance you are driving at exploring here.
Firstly, it isn't the person receiving the gift that pays - it is the estate.
Secondly, charitable gifts are not liable to IHT.
So tax is liable on the net value of the estate.
That's £500k, less the nil rate band allowance of £325k.
Net estate on which tax is payable is £175k, tax at 40%, so £70k.
So estate after tax is £930k.
Is your issue, therefore, whether Lulu gets £430k and the charity gets £500k? - because that wouldn't be a 50/50 split?