From what I have seen in the past, the ATO will use a volume...

From what I have seen in the past, the ATO will use a volume weighted average price (VWAP) of the two stocks for the first 5 days of when both stocks are trading to arrive at a percentage split of the cost base.

For example, if we take the last close as VWAP of each of the stocks as an example;

	SP				Relative
	VWAP	SOI	Market Cap		Market Value
FFX	0.2000	1,181,243,221	$236,248,644		28.91%
LLL	0.4850	1,197,538,590	$580,806,216		71.09%
			$817,054,860

Now for a share purchase of 100,000 shares of FFX @ $0.60 the calculations would look something like as follows;

	Qty	Base Cost		Date	Adjusted Cost
FFX	100,000	0.600	$60,000	1/10/2021	0.173	$17,349	28.91%
LLL pro-rata	9,681	0.700	$6,776	6/06/2022	0.700	$6,776
LLL in specie	71,429			1/10/2021	0.597	$42,651	71.09%
Total			$66,776			$66,776

Now to complicate it you will need to do a similar calculation further for each share transaction made in the past.

This is my understanding, but advise all parties to get their own tax advice.