Accessing the tax value of a marketing document row before it is written to

I am creating sales invoices using the DI and I need to check the valu of the tax as calculated by the system so that I can compare it to the value supplied on the data provided and check for rounding errors.

My problem is that none of the fields in document lines that hold the value of tax e.g. priceaftervat, nettaxamount seem to be updated before the record is written. It looks like the value is calculated as the record is written.

Does anyone know if this is the case or am I missing something?

Regards

Gordon