Splitting amounts on import

[Glenn Ramsey]

Hi,

I'm wanting to import a bank statement and use ledger/hledger to calculate the
GST (goods and services tax) and/or deductible proportion on certain amounts.

I've been playing about with hledger's CSV import feature and I can get it to
put the transactions into accounts based on the description field.

What I would like to do now is, for certain transactions, which it can identify
via regex, calculate a fraction of amount as GST and record that somehow, and
for other transactions calculate the deductible fraction of the amount.

It looks like creating a split transaction via hledger's CSV import rules file
would be a good place to do this but I couldn't find a way to get it to
calculate fractions of the imported amount.

Given that it doesn't seem possible to do this my first question is: is this the
right approach?

If not, how should this be done?

Glenn