>

>? NTOM(MTON($9)*(1/3))
>

Yes, that's correct if you want a currency data type for the result.

>This is ok for me ( and you ? ) but when Bill G. have to split they patrimony:
>

>? ($9999999999999.9999/3)*3
>?  NTOM(MTON($9999999999999.9999)*(1/3))*3
>

As I'm sure you already know, VFP can handle numbers up to 9007199254740992 (2^53) with 16 digits of precision. Working at the limit, your first example

?(900719925474.0992/3)*3

gives a correct result.

For compound calculations the best practice is to derive intermediate results explicitly. Breaking your second example into two steps

lnx = MTON($900719925474.0992) * 1/3
?NTOM( lnx * 3)

also gives the correct result.