Valuation discounts for unlisted shares / private companies. EMI Options

[dominoman]

I am trying to work out the right valuation discount to apply for private company shares in my late father's estate. The discount to cater for the fact that these are small holdings (<5% of the company) and the companies are loss making, will need more capital, and shares are not tradeable in any way.

I have consulted several people who are knowledgeable in such matters, and there is unfortunately little consensus. It seems to be a dark art.

I have been using a figure of around around 80% discount so far, but perhaps that is too low?

Take this example, for a small loss-making manufacturing company my late father has 5% ownership of:

Sept 2018 funding round Price per share of £400, discounted 80% = £80
Jan 2019 HMRC valuation for EMI Employee options: £10
April 2019 funding round: Price per share of £444, discounted 80% = £89
April 2020 HMRC valuation for EMI Employee options: £25

If EMI options are being priced at £25, is that the price that is reasonable to use? Is the valuation for EMI options generally the same as the valuation discount applied for IHT?

Thanks for any help!