Just found this, what do you think?

https://www.extremeoptimization.com/...t-Numbers.aspx

"Constructing big floating-point numbers
The BigFloat structure has several constructors that construct a floating-point number with the same value as the argument. You can start from 32 and 64 bit integers, single or double-precision numbers, BigInteger values and BigRational values.

Most rational numbers cannot be expressed exactly as a floating-point number. For this reason, a second constructor is provided that takes two additional arguments: a AccuracyGoal value that specifies the desired accuracy of the approximation, and a RoundingMode value that specifies how to round the final approximation."