This article gives an example of how to code a SOP document when the imported document only supplies header level taxes.
I'm freqently given web imports to bring into Dynamics GP, it's not unusual for them to have the taxes stated at the order level and not at the line level.
The task, then, is to back into the taxes by doing TAXAMOUNT/ORDERSUBTOTAL = TAXRATE
When you have the tax rate you can apply it to each line and build the tax import. This method will incur rounding errors, so the last detail line has to handle that.
Notes:
We bring in the DOCID, TAXSCHID and the TAXDTLID from the config file. I've never had a client want more than one tax detail ID.