[OSRS-PROJ] datums transormation and dX dY dZ values

[giorgio]

Hi all
I'm not a very expert in datums shift and projections so, eventually, I'm sorry for my basic question.  
I have a map where are drawed the grids in two different datums Gauss Boaga (Rome 40 datum) and UTM (eur50 datum). I'm not sure that parameters (dx dy dz) for the datum shift indicated in the internal datum tables or epsg or NIMA  tables are very good for my little area of work so I would like know if is it possible desume (and in which way) my user defined dx dy dz for the molodensky datum shift R40 --> ED50  directly from the two grids drawed onto map or if I need of other parameters?
Many thanks in advance.
Giorgio.  
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