Post non standard Tax Rate on FBDI AR Invoice Upload

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We have a requirement to upload invoices with non standard VAT rate percentages, but against a single VAT Code. i.e. against STD 20% VAT code, but a variable rate like 16.6%.

The only way to achieve this appears to be adding the tax line separately, rather than Fusion calculating the VAT from the tax classification. We'd prefer to enter the same line as the invoice line as its less complex, but the process fails. 

Anyone had to deal with something similar?