Want to add totally custom form that has header with drop down that populates grid

New to customizing acumatica, Trying to add a Rate Table Maintenance Screen to Accounts Receivable. Want there to be two tables 

1. RateTable with just an ID
2. RateTableDetail with that ID and a begin amount (double) and end amount (double) and a percentage. 

Want the custom form to only have one tab that opens with a blank screen.  You select the Rate Table ID from a lookup in the header section and the grid populates with all RateTableDetail Lines for that ID allowing the user to edit lines and add lines. User can add new Rate IDs in the header also 

Any help would be appreciated.