The usual estimate with the Barings value is calculated using a percentage of whatever the current gold price is pegged at. The rule of thumb value is 10 % although there is no "correct" valuation method. An extract from one article I have is :

Julian Baring, the well known City 'gold bug', used to value mining stocks
by taking, as a rough guide, 10% of the value as being the correct value for the company in question.

Gold mining rule of thumb

Curiously enough, this rough and ready guide seems to hold good today.
Figures calculated by the Mining Business Digest in 2000, based on data for corporate acquisitions in the gold sector, reckoned that those at the earliest stages of exploration, with some inferred resources, would be valued at most at around 3.5% of the underlying metal value in the ground. An average for those with more tangible metal assets to develop would fetch around 11% of underlying value while those in the early stages of production could see a value of anything from 15% to 25% of the value of the underlying ounces of metal in the ground. It is not a hard and fast rule, however, and those mining base metals are valued at correspondingly lower rates.
There is some more data confirming this gold mining rule of thumb.
According to figures compiled by Galahad Gold and sourced from broker reports and the publication Gold Stock Analyst, the average value placed on 'ounces in the ground' in 2005 was $120 for second tier companies about to start production, $50 an ounce for those with resources that had been measured and/or indicated, and $32 an ounce for those with simply inferred resources. This compares with an average gold price for 2005 of $444, making the respective percentages 27%, 11.2%, and 7.2% – not too far from those quoted earlier.

The gold prices mentioned are obviously out of date but the percentage values still apply.