taxamount on ProjInvoiceEmple Table

For a customer we need on the ProjInvoice report the tax amount on line level,

but this amount is not filled in on ProjProposalEmple or ProjInvoice Emple.

But we have seen on the taxTrans table the records are there. but we are not able to find the relation between ProjInvoiceEmple  and TaxTrans, because they all have the same voucher. please suggestion