Rounding off excise value in MIRO

Hi SAP Gurus,

          I am doing MIRO for excisable material. Process is following ME21N-> J1IEX( Capture)->MIGO->J1IEX(Post)->MIRO

          Now as per business scenario , Client wants to held J1IEX( POST) , they want to do first MIRO then J1IEX(POST)

         In this case, Modvat clearing hitting at the time of MIRO gets rounded off but if we do MIRO first then J1IEX (Post) . What I understand It will fetch

          exact value from  MIGO in MIRO in case J1IEX has been posted first.Necessary settings for rounding off is done in OB90

      ( Company Code  with Rounding Unit 100).  Please advise how we can capture exact value of excise from MIGO if we do MIRO First and then

          J1IEX(Post) and also request you to provide me the relevance of Rounding unit 100 in OB90.

Regards

NIkhil