Dear,
F-48 is vendor down payment process.Are your talking about main screen of F-48?
Ate you using EHP-4? Because SAP has come up with new solution for down payments form EHP-4 on wards, in which system it self transfers the tax from purchase order item details itself.
Report : ME2DP : Down payment monitoring report, where system will have option to take you from thsi report out put to "Down payment "process selection screen.In this option of " Transfer tax on sales/purchases code from P.O" to consider the tax structure for down payment.
Regards,
pardhu