[cc100]

Ive worked it out as:

Percentage of biscuit recipe (and therefore any amount of biscuits) taken-up by butter:
2/(2+2+7+3)*100=14.2857% (to 4 decimal places)

Therefore amount of butter (14.2857%) in 100g of biscuits is:
100*14.2857/100=14.2857g (obviously)

There's 90g of fat in 100g of butter, so in other words, 90% of butter is fat.

Therefore amount of fat (90%) in 14.2857g of butter (and therefore 100g of biscuits) is:
90*14.2857/100=12.8572g / 0.4535oz (to 4 decimal places)

Or put simply: 2/(2+2+7+3)*90